|Occupation||Physician, formerly Paramedic|
|Title||Intern (2006- 2007)|
|Family||Mike Gates (father)|
|Spouse||Samantha Taggart (ex-girlfriend)|
|Behind the scenes|
|First episode||"The Human Shield" (guest)|
"Graduation Day" (regular)
|Last episode||"And in the End..."|
|Appearances||65 episodes (see below)|
Gates first appeared in the episode The Human Shield as a paramedic who was fired at while trying to assist a young kidnapping victim. Gates transported the girl to County General following the arrest of her kidnapper. While assisting in the treatment of the girl, Gates met Dr. Neela Rasgotra.
In the beginning, Neela seems irritated by Gates, but the two soon developed a friendship. Gates later revealed to Neela that he is a medical student who is spending time with the paramedics until he graduates.
In the episode, Two Ships, when two airplanes collided and crashed in downtown Chicago, Gates and Neela again worked together, this time out in the field. Gates was badly injured when a burning building (which he had run into to save a woman) exploded. Neela was forced to perform risky liver surgery on him in the field. He eventually recovered from his injuries.
Gates' strong willed and often rebellious attitude led him to a few confrontations with attending Gregory Pratt who on many occasions called him a "cowboy"; this provided an ironic counterpoint to Pratt's earlier, similar difficulties with Luka Kovač and John Carter.
Gates reunited with Neela and encountered feelings with her. The two of them bickered with one another, but eventually had a fling which began to upset Ray Barnett, who also had feelings for Neela but could never quite face up to them.
It was revealed that Gates had a girlfriend named Meg who was using her deceased husband, Keith's life insurance money to get him through medical school. She also had a daughter named Sarah, who loved Tony more as a best friend instead of a father figure.
Gates' and Meg's relationship was revealed at first being casual friends through Meg's husband, Keith who was friends with Gates from their service in the first Gulf War.
After Keith's death, Gates decided to live with Meg and Sarah to help them out. He would eventually become intimate with Meg. Meg clearly had big hopes for him to provide more for his family. He didn't mention his domestic status at the hospital, and it was unknown even to Neela.
Gates was caught in the act with Neela when they spent a Thanksgiving together at his and Meg's place. Jealousy and fury were unleashed as Meg tried to get back at Tony and Neela. Ultimately, he left Meg for Neela, but still managed to stay in Sarah's life.
In Murmurs of the Heart, Gates gets a frantic call from Sarah saying that Meg was unconscious. Despite everyone's best efforts, Meg eventually died from an intentional overdose of medication, leaving Gates flooded with guilt and holding Sarah by his side.
The last words that Meg said to Gates before she lost consciousness was that Sarah "is yours", implying that Gates was her biological father; however, a paternity test revealed that this was not the case.
More of Gates's life is exposed when he allows his father, Mike, an alcoholic ex to move in with him and Sarah. He later throws Mike out after finding out that he drove Sarah to the movies while drunk.
Mike protested until he and Gates got into a fistfight, during which Gates justified kicking his father out by explaining that as a fire captain, he had caused a young fireman's death due to a mistake he made while drunk.
Gates begins thinking about legal intervention after speaking with his lawyer about Sarah, who wants to stay with him regardless of her grandparents' wishes; however, at the end of the season, Meg's parents show up with legal documents and take Sarah without talking to Tony, saying this will make for an easier transition.
Gates hopelessly tries to fight back but assures Sarah he will visit her as much as he can and plans to fight for custody of her.
Through the season, Gates tries to define his troubling relationship with Neela, that is increasingly strained with Ray Barnett still longing for her.
At Luka and Abby's wedding, Gates and Ray break out in a fight, drunk over the bar and over their growing hatred of one another; this led to Neela finally breaking up with Gates, as she was in the process of choosing Ray over him. Gates later tries to reconcile things with Neela, just hoping to stay as friends.
At an anti-war rally that Neela attends, an explosion broke out sending the crowd into mass panic. Gates (who was making his way through the crowd to Neela) desperately tries to get through the stampede to save her from being trampled.
Gates also loudly castigated and slapped around the guy who caused the stampede, causing an angry Dr. Moretti to ban Gates from the ER to the ICU.
Even though Gates initially was viewed as an ER reject, he bonded with a young male prodigy who has a rare and incurable disease; his strong work in the ICU and on other cases led Dr. Pratt to recommend to Dr. Moretti that he be allowed back to return to the ER.
Gates then returned to the ER and began dating a friendly and free-spirited hospital chaplain named Julia. He was later asked by Sarah's grandmother to have Sarah move back to Chicago with him because Sarah was starting to rebel against the farm atmosphere much the same as her late mother had.
Sarah's grandmother later got angry with him when he wanted her husband to have surgery after a heart episode, though they eventually mended fences when Gates says that they should go with a different option presented by Dr. Archie Morris.
Gates and Julia broke up and she went to Nepal on a long spiritual retreat. He began a sexual relationship with Samantha Taggart which started out with a kiss that Sam gave to Gates.
After Gates called Sam for being distant towards him, they began a more mature dating relationship, although they also got caught having sex on a hospital security camera.
On the first day of the ER's new chief Catherine Banfield in Season 15, Gates and the new interns treat a bio-terrorist with a broken leg who accidentally releases a bag of weaponized ricin in the ER.
Gates and the interns are forced to remain quarantined in the exam room as Banfield, Sam, Morris and Neela help evacuate the hospital. After a decontamination, it was revealed the ricin wasn't fine enough to get into an aerosol form and enter their lungs so no infection would occur, much to the relief of Sam who was worried about Gates.
After Sam's son, Alex was injured in a car accident, Gates and Sam's relationship soured until Sam made the decision to move out despite Gates continuously apologizing for letting Alex go out on the night of the accident against Sam's orders.
Gates discovered an ER patient's history as a dedicated and wounded Iraq War veteran and was able to get him some help. He later slept with intern Daria Wade and regretted it, telling Sam he was sorry about it and gently letting Daria know he was in love with Sam and wanted to fix things between them.
Later on, things between Gates and Sam appear to be improving as they are no longer fighting as much and they both are seen looking at each other whenever they are near.
In the last episode, And in the End..., he gave Sam a refurbished car for her birthday and there was a strong hint that they will be back together.