I recently just found/changed shields into one which pumped my hp up another 10k, but reduced my passive hp regen by 326 per second. In doing so I noticed two things. (Could be my imagination on these, hence me looking for confirmation, but I'm fairly certain I'm correct)
First: My pets (dogs and garg both) seem to regen hp (based on how quickly their bar refills) around the same rate. Which leads me to believe the new automatic passive regens for both are a % of max health on the pets. (which makes sense, as Blizz was trying to bring the passives up to an amount that is useful equally to those who are going glass cannon as those who focus pets.)
Second: I use Spirit Walk+healing Journey as a healing source for my pets. But I've noticed the more my max hp goes up, the less healing journey seems to help my garg's hp bar. Which seems to me, to help confirm that the Gargantuan, with the secondary 50% increase, has it's hp scale by more than 100% of the owner's. (Hopefully suggesting that the base scaling is 100% of owners, followed by the 50%. :) )
Just a couple observations, will continue to watch them myself, but thought I'd bring them to the community to look over as well.
Side Note: Those, who like me, were gearing for a CC build. Go switch over to Raid of Toads as your primary, choose your favorite Garg form (as he no-longer procs) and enjoy as the rain of toads is now completely spammable with the mana cost reduction+ new improved mana regen. Plus, rain procs fairly well.
Also of note, the rain-o-toads seems to proc more often the larger the target.... W00 for bosses being fairly large in size always. (with 3.8% chance to freeze, 2.7% to stun, and 4.5% to knockback, and using leeching beasts instead of burning dogs [will be switching back soon since I just saw that dogs DO still proc.. just really really low]. I manage to stun/freeze Diablo fairly often still. About once every 3-5 seconds. Enough to interrupt a lot of his more deadly-to-pets attacks. As well as reducing his average spike traps to about 3 instead of 6 for me due to interupts)