For NC Medic guns all of the above responses are correct/valid but at the end of the day it's up to you to try them out in VR training and trial mode to see which suits your personal play style.
I personally use the GR22 with IR/NV scope and advanced laser sight. This is because as a medic I like to be at the front just behind the people who are charging in and with the aoe heal, res grenades and the medic gun I can try to keep things moving and do a lot of shooting/c4 too. Sometimes I might switch to a shotgun or smg if I know we're going to be CQC all of the time (a bio lab say), but most of the time I find the GR22 serves me well for short to medium combat.
Other people prefer to take more of a supporting role, hang back and make sure they stay alive to keep their squad up and firing from a distance. As the outfit I'm in has plenty of medics this isn't an issue for me, but it's a completely valid way to play the class. My brother for example uses the Gauss Rifle S because he prefers to fight from more of a distance and this gun allows him to run with a grenade launcher and res grenades.
Either way I don't subscribe to the idea that medic is just there to heal and only fight when absolutely needed, with assault rifles and c4 it brings a lot to the table in combat, yes a good heal or res nade can turn a battle but you need to kill people to get into a position to do that.
So basically it really depends on how you like to play and choosing the tools to adapt to the situation you're encountering the most in the battle you're fighting at any one time.
PS, maximum level medic tool (best certs I've spent ever), medic ability (aoe heal) and res nades are all extremely worth certing into if you want to play medic a lot.