I found the best way was to keep those sticky strips like used in barns in olden days.....Still around at hardware stores. Grubs I am not sure. H.Peroxide will add oxygen to the mix, doubt if it will assist....

Gnats like the soil, so you could take real sticky tape and lay it on top of the soil. As a last resort, and if not in late flower, a pyrethrin based insecticide or shell no pest strip for 10 days should help.

Prevention is the key to these buggers. Never use unsterilized soil, nor outdoor soil. See my link for prevention and treatment measures for mites, much crosses over.