6.10.2018 0.00.17 :: Synthesized full backup was not created for 14.7.2018: there are no new restore points between 8.7.2018 and 14.7.2018
Only that dates are different.
As far as I understand how GFS works this means that for some reason in that time GFS couldn't backup anything and that is why it says "no restorepoints bethween..." which is working as designed
Now it's 10th of October so incident was over three months ago. Question is that can I get rid of that warning or do I need to wait a year so retention time goes over that date and Veeam forgot that missing restorepoint?
You need to disable the GFS job and enable it again. It should remove the warning. Before doing that please make sure all required previous backups are on tape because being disabled the job will 'forget' about all previous GFS dates no matter were those points copied to tape or not.