Gabriel are lower-end shocks, and Monroes tend to be engineered for a softer ride vs. handling and control. Put Monroes on DW's 05 Hemi Magnum and very disappointed...nice cruising on the highway, but zero control on body roll, etc. Wanted Bilsteins, but Monroe was half the price so thought I would give them a try....now I know why they are half the price LOL.
Generally when towing you want firm suspension control to resist inputs from the trailer and limit porpoising, etc., but at the same time you don't want it so stiff that you lose your fillings when not towing. Bilstein is the "gold standard" and you pay $$$ for them....I went with KYB on the Suburban based on good reviews on this and other forums, and they were less expensive. So far I am very happy with the performance. Firm but not obnoxious.