When you authorise a connection to GitLab for retrieving repository information to add to your DeployHQ project, an authorisation is created within your GitLab account.
If you wish to connect to another GitLab account, you can head to GitLab, then login using your currently linked account details.
After that, head across to your main Settings page via your profile icon to the top-right of the screen, then over to Applications on the left hand side.
You'll find DeployHQ within the Authorized applications section - just click the Revoke button on the right hand side.
Once you've revoked access, you'll be prompted to login when you next create a DeployHQ project, and you can then connect using your other GitLab account.