Hello,
Dynamics GP itself will often work with unsupported/untested versions of SQL Server, but you will definitely be version checked when you try to run Dynamics GP Utilities to perform an upgrade or create a company and you won't be able to proceed.
The SQL compatibility requirements that Richard mentioned are correct.
Since you're on Dynamics GP 2013 you can only upgrade to GP 2015 or GP 2016. Neither of those are compatible with SQL 2017, so you're only option is really to downgrade SQL to a compatible version. That is a tricky process since the databases would have been upgraded as soon as you restored them to SQL 2017 and that means you cannot restore backups to an older version of SQL.
We have done SQL downgrades as a billable advisory services in the past so if you're interested you can open a support case to begin the process of getting a quote. You could also check the Microsoft Pinpoint site to see if there are other partners who provide that service.
Lucas Miller
Microsoft Dynamics