Light brown gunk is not thermostat issues.
Thermostat's only mission is to hold back the flow of coolant from entering the radiator at set temperature.
If it was removed, engine could overheat from coolant passing through radiator not slow enough to cool off before entering the engine again.
Thermostat does not block flow to heater.
Light brown gunk can be, in foam condition,
head gasket, or
Excessive moisture, engine never gets to operating temperature to burn it off, causing acid formation.
But close inspection is necessary of cooling system.
No Heat Condition.
Reason= No Coolant in Heater Core.
Engine Coolant Level, Extremely Low.
***Disregard Coolant expansion/overflow tank.
Check coolant level at Radiator Only.
There is hand pump pressure gauge tool put in place of radiator cap that takes cooling system to its max pressure of roughly 16 pounds with engine off.
Can be forced to 20 pounds to reveal stubborn leaks, but coolant level needs to be at cap level.
Gauge pressure dropping= Is showing system leak.
Now to discover the leak.
If the cooling system has trapped air void of coolant, it could hide the source of leak.
Re-inspect cooling system level and vent any air out.
Raise vehicle by any means to help coolant fill any air-bound voids.
Best method is radiator cap being the highest point.
Make sure heater position switch or lever is in the high heat mode to allow coolant to fill the heater core complete.
Radiator only. Do not look at over flow tank.
Pre-tighten all hose clamps before testing begins.
Tool pressure applied, any fitting will show signs of leaking.
Regardless if head gasket was damaged or not, it will be damaged if the radiator is low.
**Caution on aluminum engine blocks.
Aluminum will warp at lower temps than steel.
Re-Check coolant level anytime gauge pressure has dropped.
It must be to the cap.
Re- pressurize the system.
It should stay at this level all day, not five minutes.
**Tests that do not show external leaks/
Radiator re-filled/ start the engine.
Leave radiator cap off.
Is coolant in radiator violent and bubble out?
If so, end of story, head gasket is indeed blown.
**Internal coolant leak is direct combustion
pressure entering cooling system, forcing out the coolant.
You might get lucky with adding a coolant stop leak if the leak is minor, but it' a rare 50/50 shot.
The engine needs several heat cycles to make it effective.
Pre-boil sample of coolant or water with added stop leak mixed, then add to radiator.
This is important to not have compound settle to bottom of radiator.
I would advise removing the heater core from the engine's fittings to prevent compound from entering heater core. Simply remove hoses. Use short hose to connect both engine ports.
Run engine to full temp, blocked added throttle speed above idle.
If cooling system leak seems repaired, then add heater back to the system.
Good luck on that one.
Forty Years in Monster Garage, where guessing is not an option.