Hi there
How did the Gammarus sp. get introduced to your tank? they usually inhabit fast-slow moving bodies of water accross the UK (or wherever you might be).
They are sometimes a good indicator of water quality, especially in terms of lack of chemical/metal/agricultural waste.
They are fairly harmless, will scute along the bottom lmainly, and like places to hide. they prefer a vegetable based diet, and mainly feed on rotting leaves.
The only thing to consider is, they are disease vectors (for your fish/inverts) and if they were 'wild caught' it might be worth eyeing them, up for visual parasites etc. before introduction.