It's on the same side of Gran Via and you can walk to the hostal without crossing any streets
That's true for one of the four exit/entry points to that station. Hopefully, someone here is familiar enough with the Gran Via metro stop to know the exact wording of the signage indicating that specific exit. That's really what you're asking for.
It might be an easy matter for the able-bodied to "walk more than 2 buildings out of [their] way" to see if the numbers are going up or down but for someone walking painfully with a stick and carrying luggage it's quite a different matter.
Not "more than"--no more than.
However, if exiting the metro from other than the very closest point will present a true inconvenience, perhaps consider these options:
--take the metro to the closest station with accessibility (here's a list--sorry, not sure if you'll be taking the No. 1 or 5 line) and take a taxi to the door.
--consider accommodation closer to a metro stop that has accessibility.
I don't know if all metro stations have personnel on site. If this one does, simply ask, "Salida sureste?" and they should point you to the desired exit--though there are no guarantees.