A bit late to this one but adding the practical version in case it helps the next person searching.
A GL code lives in your QuickBooks chart of accounts. Jobber's accounting codes are labels you create yourself, so they only line up if you name and map them to match your QuickBooks accounts. The code doesn't do anything on its own. The mapping is what does the work.
The catch specific to expenses: the sync moves clients, invoices, and payments, but expenses don't come across at all. So the accounting code on a Jobber expense is really for Jobber's own job costing reports. For QuickBooks, your expenses still get entered there separately, usually from the bank feed or receipts.
If you're setting things up fresh, the easiest order is: finalize your chart of accounts in QuickBooks first, then create Jobber codes with the same names, then double check the mapping in Jobber's QuickBooks settings. Renaming accounts later is where most mismatches come from.