Hi,>>add a water temp gauge.
I look at that gauge every 10 minutes while the engine is running, and I'm not the only one. It's a habit, especially when running anywhere near Wide Open Throttle. The relative readings over time let you know the rate at which the cooling system is gumming up from scale, hoses and the passage of time.. The formerly expected 180F becomes 185, then occasionally higher, then eventually pushes 200F. More a message than a warning.
It is a comfort to monitor the running temperature. A warning light is OK, but not a comfort.
Sometimes an owner will run without at thermostat if the existing one fails in a closed position. Running without a thermostat makes it harder for the engine to operate at its normal temp, but otherwise I don't think it causes any other problem or damage.
The mating surface in your pic looks ok far as I can tell, as long as any rough spots are removed. I only use the gasket, no sealant. If it doesn't seal properly, ie. it leaks, then a gasket sealant may help, but I would try without first. Tighten the bolts, but not so much as to risk breaking them off, which creates a whole new set of problems.
In not a mechanic, so just going from my experience.