Hello Alex32828 and welcome to the Community.
The reason you have a 3/4” air gap is that in order to hang the Drywall they installed 1”x 4” furring strips, to screw the drywall to.
To get a well-secured
cabinet you may want to remove the drywall, which will be behind the mounting
strip of your cabinet. Then mount a 1 ¼”
thick board horizontally across the wall. The problem
with this is, there is not a 1 ¼” thick board.
Therefore, you may have to sandwich 2 pieces of wood together with wood
glue, for instance, a deck board and a 1/4” strip of plywood.
The actual thickness of a deck board is 1" and the plywood is 1/4".
You would then use flat head 1 ¾” Tapcon screws in a “W” pattern, to mount wood to your concrete.
Now you can mount your cabinets to this well secured mounting board, with GRK self-tapping Cabinet screws.
Thank you for your inquiry.
Charlotte