GTA 5 references it a few times, but the casual fan might forget that Trevor Philips is Canadian.
Trevor Philips was born and raised in Canada. It's not an integral part of the GTA 5 storyline; hence, it's easy for a casual fan not to remember it. However, the game still references Trevor Philips' nationality several times throughout the game.
The state of San Andreas is based on California. Both Franklin and Michael are American, but Trevor is not. Interestingly enough, Steven Ogg (Trevor's voice actor) was born in Calgary, Canada.
Trevor Philips even references his "faint" Canadian accent a few times.
The nationality of GTA 5's Trevor Philips is Canadian
In the mission, Friends Reunited, Trevor Philips talks about his Canadian roots in a story to Wade as they head to Los Santos. Trevor describes himself as:
"He was the smartest, toughest, weirdest kid in Canada."
He later specifies it was at the Canadian border near the United States and then talks about how he didn't know what to do with his talent. Trevor's backstory even includes an inappropriate moment with a hockey stick and a coach.
Trevor Philips signed up for the Canadian Air Force in hopes that he could bomb villages as he had talent in flying planes. His high-flying skill references this trait at the start of the game.
Unfortunately for Trevor Philips, he fails his psychological evaluation. From there, he eventually meets Michael, the GTA 5 Prologue heist ends disastrously, and he ends up in the northern part of San Andreas.
Rampages in GTA 5
Several in-game rampages further reference Trevor Philips' Canadian nationality. The Vagos, military, and hipster rampages all discuss his Canadian accent.
In the Vagos Rampage, they ask him where Trevor is from before he goes on an outburst about how it's a slight accent. He later reconfirms that he grew up in Canada with the following quote:
"Hey, so I grew up in Canada, huh?"
The military rampage has Trevor say one word ("Nothin'"), and the soldier makes a joke about how he's a tourist from up north. The soldier proceeds to make more jokes with an attempt at a Canadian accent. Eventually, Trevor Philips gets infuriated and takes a gun from the back of the soldiers' car.
Finally, the hipster rampage in GTA 5 also talks about Trevor Philips' nationality. Trevor confiscates a phone from a random hipster, so they argue for a bit. The hipster then states:
"If you don't like it, you can go back to Canada, dude!"
More references
A random soapboxer known as Griff talks about Canadians. In this instance, Trevor brings it up, asking Griff what he thinks about them. Unsurprisingly, the minor character does not like them and is blatantly xenophobic.
If Trevor hangs out with Franklin, he might even tell him that he:
"Served time, my country, your country and myself."
In this case, "my country" refers to Canada.
Michael even pokes fun at his nationality a few times in GTA 5 while the two hang out. It's one of the few times that Trevor Philips doesn't try to eliminate somebody for referencing his Canadian upbringing in GTA 5.