Unable to backup to a UNC path with a gMSA
If you check the documentation for Ola Hallengren's solution, you'll see under "Which permissions are needed for the SQL Server Maintenance Solution to work?" the following:


  
  DatabaseBackup: sysadmin
  DatabaseIntegrityCheck: EXECUTE on dbo.DatabaseIntegrityCheck, VIEW DEFINITION on dbo.CommandExecute, VIEW DEFINITION on dbo.CommandLog, VIEW SERVER STATE, db_owner on all target databases
  IndexOptimize: EXECUTE on dbo.IndexOptimize, VIEW DEFINITION on dbo.CommandExecute, VIEW DEFINITION on dbo.CommandLog, VIEW SERVER STATE, db_owner on all target databases
  


I believe the reason sysadmin is required is because the solution uses xp_fileexist to check the existence of a directory, and this extended stored procedure requires sysadmin rights.

Normally, the jobs are executed by a SQL Agent which has sysadmin rights by default granted to the per-service sid NT SERVICE\SQLServerAgent (or NT SERVICE\SQLAgent$instance_name for a named instance). If you use a proxy to run the jobs in SQL Agent, that proxy also needs sysadmin, as does the user account running the procedure from any other scheduler, as is your described scenario.