Did Vasquez and Gorman know each other?

Vasquez's comment was gallows humor in the form of a dry, bitter complaint about Gorman's failure to perform. In other words, "Gorman, you've failed me again." First Gorman led the team into the Alien ambush and then panicked, forcing Ripley to take charge and extract them. Then Gorman got injured, leaving a noncom in command. Finally the team got into difficulty in the med lab and Gorman again failed to get the job done, running out of bullets before killing all the pursuing Aliens.

No, Vasquez didn't know Gorman before the mission. The comment was to show she accepted him finally as one of the team. He'd screwed up plenty, but he wasn't a bad Marine (just an inexperienced leader, probably deliberately chosen by Burke to fail), and she figured at the end when he came back for her he was "one of them" instead of how he'd been seen earlier in the movie ("looks like the new lieutenant's too good to eat with the rest of us grunts"). You can see it reflected in how Vasquez gives the "double hand grab" earlier with Drake, with whom she was closest in the group - she does the same to Gorman while he's holding the grenade. The banter between the Marines in the movie shows how they could make fun of each other but of course had trust in each other.

"You always were an asshole, Gorman" is shades of humor, acceptance and camaraderie from her in their last moments.

At the end of the day, Gorman's incompetence earlier wasn't a reflection of his true character, and she accepted that. The aliens were a formidable foe, and pretty much none of them were prepared for it.