You will have to lift the membrane and the gravel if for no other reason than you need to prepare the soil underneath for turf - scrape the gravel off with a rake, then pull the membrane up, turn over the soil underneath (doesn't matter if some gravel gets mixed in, that's fine) and prepare it properly for turf, which means digging it all over, then walking over it closely with your heels to compress any air pockets, then lightly turning it over again and raking it level, at which point its more or less ready for turf. I'd also add fish blood and bone as a fertiliser for the soil - sprinkle it all over before you start digging. When you've prepared the area properly and you're ready to turf, sprinkle Growmore over it and then lay the turf.