How does DeGray's Pilebunker chip work when trashing Gems of size 2+?
You don't gain anything!

Pilebunker's actual wording is:


  Opponents reveal their hands, trash their largest [Gem chip], then gain that many [1 Gem chip]s.


Note that it's "then gain", not "then you gain" - each opponent gains the 1-Gems. 
Pilebunker effectively splits their biggest gem in their hand into 1-gems, diluting their deck.