Overheating

I would have a shop check to make sure that the head gasket is OK. This would explain why the coolant is disappearing and the overheating.

You most likely have a leak probably in the radiator itself or in one of the hoses now it could also be just comming out of the over flow on the resevior or the box you put coolant and water in it should have a cap that says coolant only look near the top of this there will be a hole for water to over. try looking under your car and see if you can see where it is leaking from. 2. as for the temp. gauge you will probably need a temp. gauge sensor they are fairly cheap probably $15 - $20. This should fix that problem.