Cliff, I'm not familiar with GE's - otherwise I would have you round for a cuppa and a look around. I can however offer an educated guess.
That spray is indeed coolant that's dried up, I had the same substance appear when my rad pipe was leaking.
Are you getting heat from the blowers at all? Reason I ask is that the thermostat that controls coolant flow into the nether regions of the heater system can sometimes fail with age. Does the temp drop as you put some load on the engine?
The expansion tank's purpose is to 'pull' coolant into the engine as it cools after switching off, as a result keeping air out of the system and filled to the brim. If it is emptying after you park, then that would explain the gurgling. This would suggest that the rest of the system is on the dry side.
Another simple and common problem associated with overheating can also be a buggered rad cap, which actually helps regulate the coolant pressure in the system.
Do have a look under the rad cap and tell us what you see.