In Accounts and Sync, the Mail settings appear to affect Google mail - which I found odd...
I had all Google services set to sync and the separate Mail (which I assumed was for HTC's mail app) with Sync switched off. Reapplying this fixed a GMail sync issue (I.e. no notifications).
Previous Phones: HTC Google G1, HTC Hero, Nexus One, Nexus S, HTC Desire HD, Samsung Galaxy SII, HTC Sensation, Galaxy Nexus, HTC One X, Nexus 4, HTC One
Previous Tablets: Motorola Xoom, Nexus 7.
Current: Nexus 5, Sony Tablet Z LTE.