While I'm no expert on what body temperature a gabf should have it will always be cooler than yours thus always feel cool. If your substrate is 85 degrees I can only imagine your cooking the poor frog which would be why he is hiding under a log.

I keep one side warm with air temperature at about 85 - 90 and one side cooler at 75 -80 depending on the rooms ambient.

Too warm can potentially be as bad as too cold, what are the air temps in the terrarium?