Is it worth keeping at the start of the game or can I sell it right away ?

I think it is just a prologue weapon, an early and weak gun, rather than anything worthwhile.

I'm holding onto it for nostalgia and for giggles, but I think it is safe to sell it.

Is it worth keeping at the start of the game or can I sell it right away ?

Sell it, it really is just junk (the hunting bows on the hut dwellers are better weapons).  There are a couple pistols lying around down the road a piece past gilded vale, though pistols are a bit...eh.