Interesting that when tanks go into hibernation or neglect, the water and substrate are not always removed Can't imagine moving a good size tank without removing all that weight.
Anyway, my tale is of a 90 gallon left in place, with lights out, heater off, water left to evaporate only adding water only to hide white stains from calcium lines on glass. Anubias plant lived, and is currently growing in same tank today after bringing tank back to active use. To the best of my knowledge, the plant is 20+ years old.
CH
ps Is it possible that guppies could survive in moist gravel?