Is this gib adjusted with setscrews (the cheap way), or with a full length tapered gib which adjusts via push screws at the ends?
Supposing that it is the former, you could have one setscrew creating drag pressure on one portion of the gib, while the others are too slack. The best way, IMO, to make a solid feeling slideway with that type of gib is to shim behind it with brass shimstock. You should be able to find a combination of shims that will be within .001 of being perfect. Then, you don't need to rely on the uncertainty of tightening and locking all the adjustment setscrews perfectly equally. You will still need them in there to keep the gib from falling out. Drill through the shimstock so the setscrews can engage the dimple in the back of the gib.