This is a dicey issue depending on how many users are in the application at one time. Normally you would do this:
Get the value first:
iJobNo = "SELECT Top 1 myTable.JobNo FROM myTable WHERE [JobLocation] = '" & Me.JobLocation & "' ORDER By JobNo DESC"
Me.JobNo = "GW0-" & iJobNo + 1
I might put this on the Before Insert event, but not on the combo's After Update event since if a user opens an existing record and flips the combo by mistake, then you will get a new job number. Unless you had a new job data entry form that had some other controls to prevent this from happening or being opened to view existing records. The Before Insert event happens once per record without controls, but the JobNo is not generally available until after the record is created and navigated from.
Did any of that help?
Look it up at: http://wrox.books24x7.com