I haven't used the Gabriel's, but I did run Monroe air shocks at one point.
They help to level out the vehicle, BUT they do so by taking the load off of the leaf springs and instead putting it on the shocks...specifically the frame crossmember the top of the shocks mount to, and the bolts for the lower shock mounts.
Such things are probably okay for vehicles that carry light loads and where the need is to level out the body for highway towing (aerodynamics, visibility, etc.) without buying a whole new rear suspension.
However I don't think they are particularly safe for heavy load bearing, and functionally, they cause the shock to try and do something it's not designed for.
I switched to WAR123 leaf spring shackles for a small amount of lift in the rear, swapped out the leaf springs from left to right to get rid of the left side lean, and went with Bilstein shocks. Now the rear carries weight even better than it ever did with the air shocks, with very little if much sag even when loaded with gear and a trailer, and the shocks do their job as shocks and dampen the suspension/body movement.
My suggestion would be to get good shocks instead, and look into swapping the rear leafs and maybe a tiny (1/4"-1/2") lift with WAR123 or aftermarket/custom shackles.
There are also "helper" springs available as a bolt-on to supplement the rear leafs when towing a heavy load. These may be just the ticket since they are inexpensive, and can be attached for the winter or when you'll be towing, and are removeable if needed.