uhhh I played a gunslinger engie on fastlane today, got a decent 51 points
I wouldn't do it unless you already have an engie, as everyone will bitch about not having a teleport
camping gunslinger engie is useless, you have to move constantly=stuff is vulnerable to sapping
throw a dispenser near the wrench engie's nest, more metal for him plus it acts as a sapper marker for you(you might be able to save the nest)
remember, engies are bros, if you aren't going to set up teles and stuff, at least help the other engie who does.
use pistol+shotgun as you'll want to support the mini instead of letting it kill
don't be afraid to fight, you have both short and long range power and good hp, with a mini you can even 1v1 a heavy if you catch him off guard
also don't do it in campy modes, it's pretty much 5cp and arena only.