Have you looked at your coolant? Is it red/green or whatever color it’s supposed to be? If it’s discolored I would do the recommended flush and replace thermostat regardless.
When your 4Runner is cold and you start it does the coolant temp in the dash slowly get warmer or go from fairly cool to hot family quickly, it may be cool for a while before your tstat opens? If it warms slowly from cool that indicates the thermostat isn’t working as it should.
Does your engine oil look good, or like a milkshake? Do you have white steamy exhaust? Are you losing coolant or it’s just not cooling your engine? Just wondering where they are determining the head gasket.
Sorry to hit you up with so many more questions.