Sell it outright. That is a good chunk of change and the price on the GT is too high.
If, after some time, you have find you want more than just the GSA then consider buying a used GT. You can always buy a newer one later on if you so desire.
The GSA is a great all around bike and there is no shame in being happy with it.
2019 BMW R1250GS
2016 BMW S1000XR