Sam Witwicky: [to Lennox] We gotta get Sentinel outta here, he is the key to all this...
Sentinel Prime: Indeed I am!
(transforming noise) What you must realize my Autobot brothers, is we were never going to win the war!
For the sake of our planet's survival, a deal had to be made...
... with Megatron!
Ironhide: [rusting away] Sentinel... what have you done?
Sentinel Prime: [to Ironhide] I hereby discharge you from duty!
So in my head canon Sentinel has betrayed the Autobots so that when he combines he becomes a Decepticon! The Autobots he combines with have no memory of this....
To me it's in line with Dark of the Moon
The gestalt head is a FoC Ruination head with the antenna removed.