Google just notified me of what I already knew: the FEDs are reading my emails (and my DM's and other electronic communications). Why? Because I will not be silent about the rampant corruption within the U.S. Government. We must end it. 
Discussion
And because a guy in Belize died because of your girlfriend.
As a matter of fact, the US didn't care about the allegations from Belize. McAfee returned to the USA after fleeing Belize and was left alone for years until the IRS allegations.
Apparently the IRS only goes after white collar criminals after they turn into white trash criminals.
Wow why on earth would you have a Google account? Tracking the trackers?
He believed secure email services were owned by the CIA.
There is precedent for this (intelligence services selling backdoored encryption products) and it's impossible to verify the security of encrypted email services independently.
So basically the alternatives are not better is what he believed and that's why he used Gmail.
Presumably he was also aware of how to use PGP, which is better than just using ProtonMail because PGP run locally is client side encryption.