I don’t know if there is some other lore about them (in comics, games, or novelizations), but in the theatrical cut of the movie, according to dialogue, Gorman is a brand-new lieutenant, straight out of cyro-sleep. His only woken relationship with the unit (Hicks, Hudson, Frost, Ferro, Apone, Vasquez, et al.) is from the few hours spent on LV-426.
The problem with the line is the phrasing ‘always were’. That implies long-term dealings. The put-down “You’re STILL an asshole, Gorman” would have made more sense.
Granted, the spoken line is softer, a sympathetic balm for someone who’s getting ready to eat an incendiary grenade with her—but when else was Vasquez soft?
My interpretation: "You're still an asshole" would make sense if Vasquez was looking to simply put Gorman down, but I don't think she was. There is some earnest, if grudging respect behind her final words. Gorman made a lot of mistakes, but he put himself on the path to redemption when he bravely and selflessly decided to go back for Vasquez and, when realising they weren't going to make it, provided the means with which to end things on their terms. The line "You always were an asshole, Gorman" communicates respect and camaraderie while not veering into sentimentality, and is perhaps precisely the praise Gorman earned in the end, no more no less. Basically a way of saying "I guess you had it in you after all", or "It's been an honor" when it really hadn't and neither of those would sound right coming from Vasquez.