The Monster Gaiters seem to add about twice as much as the Beast Gaiters. As Hume BST75/WHM, MND64, Pet Food Zeta heals 407HP. With Beast Gaiters, 447. With Beast Gaiters and Ogre Gloves+1, 492. With Monster Gaiters and Ogre Gloves+1, 533.
The Beast Gaiters numbers are a little odd, but it appears that each "Enhances Reward Effect" gives about +42HP to Pet Food Zeta, and that the Monster Gaiters give two levels of it, which would explain where the "Reward II" came from before.
Incidentally, each point of MND increases Zeta Reward by 4HP, so Suzaku's Sune-Ate (+60HP) are better for Reward than Beast Gaiters (+42HP), but not as good as Monster Gaiters (+84HP).
Also, Apollo's Staff doesn't give a 10% bonus to Reward, it only increases Reward by 8HP because of the +2 MND.