First of all, welcome to RV FOrum - glad to have you join us!
The GVWR has little relation to the towing capability. GVWR is for weight carried in/on the van, not pulled behind it. The number you need to consult is the tow capacity, the amounbt that can be pulled along behind it, which is derived from the GCWR (Gross Combined Weight Rating).
The oldest tow rating I could find for an Astro was for a 1999 and that was 5000 lbs. If your 1991 is anywhere close to that and in reasonable operating condition, your pop-up will be well within its capability.