Not that isolated as you think, i did find another guy, whose account was hacked 25 days before mine. The chances to be my own mistake are quite low, a check with 4 antiviruses reviled nothing. Chrome history looks OK, my extensions have been verified, my Gmail has no trace of an external device.
My first ticked i opened with their support was me, explaining the problem, and asking them for t a list with recent login ips and informations. Guess what?! They ignored it !
This might seem like a very silly question, but how secure/robust was your password. Did you have a unique password or did you reuse your password on other websites? Given that you did not use 2FA, your password would have been your only defence.