Everything you've asked so far is covered under the HELP tab; the FAQ covers Genetic Power Level a.k.a. Descendant of Heroes (they're the same thing).
The skimmy is 2 level 50 pets bred together produce a baby with GPL of 1. 2 pets at level 50 with GPL 1 (both of them) produce a baby with GPL 2. 2 pets at level 50 with GPL 2 (both of them) produce a baby with GPL 3. That's the max. Each GPL level gives +10 to ALL stats (60 stats in total).
This is all covered in the FAQ already but if you're still confused then just say so.
The buildings are just that, extra loot if your pet has the GPL required.
As for the beast temples, they're extra-hard "boss" type mobs that give lots of good loot when killed; but you'll need pets with REALLY good genes and/or GPL to beat them.