it all depends what you want to do to be honest, but here are a few basics :-
since its a G&G clone, the madbull Daniel Defense Rail will drop straight on. that will give you some nice rail space up front for a foregrip. Grip pods can be had off ebay for £10
ant supplies do various optic adapters, including a 19mm(iirc) rail to 20mm ris adapter, and an ACOG adapter (the sight rail on the gun is either 18 or 19mm on the G&G so i think the army one is the same)
then for lighting there is the element LLM-01, looks great, eats batteries (get some rechargeable cr123's)
hope that helps