I'm inclined to say that you can leave it as it will act as a weed suppressing membrane - my only doubt would be whether it would stop water filtering through as standing water under your gravel could lead to slime developing.

Agree with Urbanite. Don't know if you have them in the UK but laying gravel ontop of a non or slightly porous surface is an invitation for the whole area to be infested with very small biting midges. They are commonly called "noseeums" and can drive one crazy with hundreds of bites in a very short amount of time.

Yes, you should remove it first. For one thing, you don't know what its been laid on - most of the time people don't do it properly (or the contractor doesn't) and just lay it on sand, when it should be on granite dust. Laying it on sand means it tends to move and shift over time, often ending up like a rucked up carpet, and that's the last thing you need under gravel. Remove it, laying instead a reasonable weight geotextile fabric or membrane, not the thinnest weight one freely available in garden centres, overlapping any joins by 8 inches, pegged down with staples (like tent pegs) to stop it moving and lay your gravel and stones on that. This will drain more freely and quickly than laying stones over artificial grass.