Gmail app is built for Gmail, no surprise here, and is able to leverage a bunch of things Google side which other apps can't.
Holding a network connection by itself is not bad, IMAP IDLE is somewhat expensive in other ways, so -- if you're seeing very high, out of proportion, "mobile radio active", then it may be the same 5.0 - 5.1 issue again, may not be fully fixed in 6.0.
By itself, the app's (any app's) "mobile radio active" time should not be larger than "keep awake" time, or maybe somewhat larger, but not a "multiple times" greater (don't know if that's what your numbers are, so just in case...)