Go into Explorer Section, select your environment that you wish to remove the permissions for, and click Remove Permissions from the quick actions panel on right side of the screen.
In the next window, pick your user and select Remove from Site Collection Administrators. Once applied, you have just removed those access to your user account across your environment.
Keep in mind that you won't have sufficient access rights to run reports in Sharegate anymore, nor structural migration as expected.
If you need more details on how to remove permissions, see here.