If you mean the level of your slab are out by half an inch that becomes a lot more at the apex of the lower end.
Not knowing what type of GH Aluminium or wood and how you intend to use sealents on the base.
You could make up the level with cement by resting the frame on to a thick mix (not to much water) and level it off using a spirit level and length of wood to tamper down on
Another way is to put the GH in situ then level it up with wedges then pack the cement in and leave it to set, remove the wedges then pack the holes.
The problem will be drilling into the slabs to screw it down.
If it aluminium and i had spent that much and had the time i would build a level breeze block base so the frame ready for the frame to go on then the holes are easily marked and drilled by moving the frame. This will also give you that bit extra height