If the job finishes within the period when a new GFS flag must be assigned, Veeam Backup & Replication checks whether the backup job has created a full backup file.
Yes: If the GFS flag has already been assigned to another backup file during the scheduled period, Veeam Backup & Replication does not assign another GFS flag to the backup file. If the GFS flag has not been assigned, Veeam Backup & Replication assigns it.
No: Veeam Backup & Replication waits for a full backup file to be created.
Does it mean that we're unable to use GFS in Forever incr backup scenario's without Synthetic/Active Full backups configured?
This is correct if we are talking about GFS in primary backup jobs. As mentioned in the limitations section of the User Guide you linked: "You must configure your backup jobs in a way you do not lose any essential data due to an insufficient number of full backup files. For example, if you configure monthly GFS retention, you need at least one full backup file per month.".