CRS is an old term from when it was possible to take reservations without a computer. It can also mean "central reservation system", in the sense of "we operate above the level of a single property, aggregating all supply for an entire chain/brand".
GDS was the system travel agents use to access supply (lodging) that's been aggregated all in once place. Its importance has fallen as fewer people are using travel agencies, and OTAs (online travel agencies -- Expedia, booking.com) have created separate pools of supply.
I work in this area. If you need any help, questions, consulting, etc., email is in profile.
