To give Sessionlimit Database access to gmsa or any account as dbowner, you can use the script specified below. You should change DOMAIN\GMSA$ with your domain and credential information.
USE [master]GOCREATE LOGIN [DOMAIN\GMSA$] FROM WINDOWS WITH DEFAULT_DATABASE=[master]GOUSE [SessionLimit]GOCREATE USER [DOMAIN\GMSA$] FOR LOGIN [DOMAIN\GMSA$]GOALTER ROLE [db_owner] ADD MEMBER [DOMAIN\GMSA$]GO