There are only two possible options; either Gannon removed himself or the club removed him.
The club said:
"This has been an extremely difficult decision for the Club’s Board to take and not a decision taken lightly. The decision is not results based but is centred around culture."
What decision were they talking about if not the decision to sack Gannon?
If Gannon left of his own accord then there is no decision the club needed to make.
I accept it doesn't say the word 'sacked' but not many sacking announcements by any club ever do; I think it's pretty obvious what happened.