There's a website that does this sort of thing across GamersGate, Steam and GOG.com.
http://didimatic.com/dd/en/onsale/
It lists savings, regional pricing, restrictions, and DRM for titles on sale.
I don't know why Impulse, or Direct 2 Drive are looked over. Not that I visit those others very often as they tend to have rather stringent region locking (Us/Canada only deals).