Dol Gam is pretty handy for the pub iirc, and so is the site in Nant Peris right across the road from the Vaynol Arms. Bu both come under the pretty basic tag.
For better facilities there's the Forestry Commission / Camping and Caravanning Club site a couple of miles outside Beddgelert, but then it's not particularly well placed for walking to a pub.
There's also a site in Betws y Coed, over the footbridge from he Railway station - it's been a few years since I stayed to I can't be sure of the facilities but i think they were less basic than the Capel / Ogwen Valley sites, and again it's handy for the pub (Royal Oak next to Cotswold in Betws).