Yes, it is possible to make purchases on arrival Terminal 1 (including transfers). However, you must show your valid boarding pass when making your purchase. Please note that it is not possible to access the shops at certain departure halls.
GASSAN Schiphol can only be found on the airside. So you can only make purchases when you are going to fly.
You can find the rules for this on the website of the Belastingdienst. We advise you to read the applicable provisions carefully.