I would personally try for either one tooth bigger at the front or 3 teeth smaller on the rear. I do this for the summer on my DR 750 and the fuel increase is significant as cruising at @ 70mph is more rerlaxed and less revvy, but then that's a 750.
BUT I usually do carry a lot of touring gear.
I use 3 teeth smaller on the rear as the front sprocket has a guide which runs closely to the chain and would not facilitate a bigger front sprocket.
I just may try a 1 tooth smaller front sprocket on my SRX to see if it will rev out better in top!
SteveD
The thing to bear in mind with the GB is that it has a different power delivery from the DR - peak "power" is made at over 7000 revs. Whether raising the gearing is a good thing to do will, in my experience, depend on what "relaxed" cruising speed is being aimed at. If you want to do 70, then wind resistance isn't going to be much of an issue and it's probably worth doing. If, on the other hand, you want to get to the upper reaches of the GB's speed range (ie 90-100), raising the gearing could have the effect that you can't overcome the wind resistance to be going fast enough to get into the power in top, in order to maintain your speed and avoid needing to drop down into fourth again. That's been my experience with the 500 engine in some wind conditions, even on standard gearing. If your engine is tuned in any way, then disregard the preceding ;-)