No, they don't really get used to the needle, but it isn't overly traumatic for them, either. Be sure to warm the injection (I hold the filled syringe under warm tap water for two minutes, then test on my arm) first and do be prepared for how tough a guinea pig's skin is. You really have to punch the needle through, which is different than any other animal I've done an injection on so it's a little scary. If they give you a choice at the vet clinic, I'd not recommend getting syringes that are any smaller than a 23 gauge because in my experience they are big enough to get through the skin without much issue and can still push the fluid pretty quickly, as opposed to a smaller gauge, which you'll struggle to poke through the skin and then it'll take forever to push the fluid. Personally, I prefer a 19 or 21 gauge because they are very easy to insert and pushing up to 60 mL at a time doesn't take too long.
I had to give subQ injections of an antibiotic and foolishly thought I could use a 31 g insulin syringe - it wouldn't even poke through the skin, but trying left both me AND the poor pig traumatized.
Good luck, please let us know how it goes!