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, Google or LinkedIn accounts.
The account you use to log into Gearset is only used to create your Gearset user profile. If you log in using a Salesforce account, we won't deploy to this org or read the metadata from it (unless you separately add it as an org connection). 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 Salesforce sandbox orgs 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.