It doesn't really matter so long as you always use the same account. Logging into Gearset is all done via OAuth, using one of your existing Salesforce or Google accounts.
The account you use to log into Gearset is only used to create your Gearset user profile. We won't deploy to this org, or read the metadata from it. We also don't see or store your user credentials (e.g. password) - that is all held by either Salesforce or Google and securely managed via the OAuth process.
Pick an account you're likely to remember using, and one that won't be deleted. You'll use this account every time you want to log into Gearset.
If you accidentally log into Gearset using a different account, let us know. We will deactivate it for you so you don't end up with multiple accounts running in parallel.
Note when using sandboxes: when sandboxes are refreshed, your Salesforce user account is sometimes deleted and recreated. This can mean you'll lose access to your previous history in Gearset, as your Gearset account is tied to a specific Salesforce user. We recommend you don't use sandboxes for paid Gearset subscriptions.