If the frame rails are strong enough to lift the machine,it could be quite easy.A pallet truck and a couple of machinery moving skates would probably do it.You may be able to hire skates for the job.On the gravel drive it may pay to get hold of some scaffold boards and use them as tracks.I hope for your sake the drive is near to level as a steep slope would be a nightmare.Securing the gantry and the Z axis may be prudent.

If the lower frame sections are a bit flimsy you may have to use a crowbar and a pile of plywood packers to get the machine off the ground and you can -very slowly- roll the whole thing around on short lengths of metal pipe or scaffold pole.Don't rush and don't put any part of your anatomy in places where it might get crushed.