I'd remove it - if it were a loose, thin gravel layer, I'd not worry about it, but a thicker layer that's been compressed isn't great. You don't need to remove every scrap of gravel, but you do need to break up the surface of the soil where its been laying compressed to aid root penetration of the turf. So, scrape off most of the gravel, fork over the soil, level off, walk all over it to get out any soft spots, then rake it again, topdress area with Growmore and a bit of topsoil or compost mix, lay the turves.

thanks very much

Is the gravel area natural or an old driveway. If natural it could be very deep as in the New Forest. Break up the hard surface, put down a landscape roll, put on at least 3 inches of soil, turf is not very deep rooted. If an old driveway, care should be taken as you may bring up the hard core base. Do the same as above, but add 6 inches of top soil, buy in fertile soil and you will need no fertiliser when laying the turf. Feed the lawn later, turf grows mainly on the top inch of soil. The turf is always well fed before it is lifted.