-10 pips: Gross or Net?

Let's say I stack enough DOT's on a monk to cause 10 pips of health degeneration. The monk, belonging to the most savvy of classes, wisely casts Healing Breeze so that he's now only losing say, 5 pips instead of 10. Could I bring him back to -10 pips again by casting Conjure Phantasm? This would mean I have -15 pips gross health degeneration on him however only -10 net because of Healing Breeze.

I guess I'm asking is the cap at +/- 10 pips a gross cap or a net cap?

Yes, the cap is applied after all other calculations, so you can indeed stack over 10 pips of degen to offset Healing Breeze of whatever. You can even do it before the fact (you don't have to wait for Healing Breeze to be cast, you could stack 18 degen ahead of time, causing an up to 8 regen breeze to simply have no effect when cast).