Did you think about wrapping the gatekeeper back on itself above the frame buckle? Wouldn't be as tidy as adding the loops, but should work.
You could get 2 sets of the accessory straps and substitute for the gatekeeper straps, but that runs $50 plus shipping, which is insane.
Best thing to...
NICE frame with Mountain Ruck is 20 inches, so hard to get any load lift unless your torso is very short. Special order load lifters from MR run $80. Frame is also 2lbs heavier than Guide light MT. Also no load shelf feature. Will be indestructible, though.
This also works with the MR Daypack Lid if you get those same split bar buckles. I got my buckles from Kifaru. You can also rig something up with Kifaru clips.
The regular MOLLE ruck doesn't appear to have a top sleeve/cap, but the MOLLE 4000 airborne ruck does, I think, to fit on a Down East or n ALICE frame. Should work on a NICE frame.