Temp gauge goes to 260 and truck not hot - 1994 GMC Suburban

SOURCE: 1988 gmc sierra temp gauge

you could have an air lock in the system. Run with rad cap off and cycle the thermostat, adding coolant as required. Or a possible heater core.

SOURCE: Temp gauge went to 260 & stayed there. What could be the problem?

The only real way to answer your questions is for you to open the hood and look it over. Once it has cooled down, re-fill the cooling system (use water...it's cheaper and to do repair you will need to repalce it anyhow...Once full carefully look everything over to find where the leak is.Also make sure that the drivebelt is there. (from what you said, likely it is) If it's water pump, you will see coolant running out from under the pulley. Once you have looked everything over, if you have not found leak, start engine and close the cap...look again...as pressure builds, small leaks will become more obvious. If you don't find leak, remove the thermostat and change it.
After changing stat, if problem persists, it's likely that you have either a plugged radiator or an internal engine gasket problem. If you have any auxiliary electric cooling fans, make sure they are working and also check the fan clutch on the mechanical fan (it should not excessively freewheel when hot). Whole testing, any time you replace the radiator cap you must allow engine to cool down before removing it again. Watch temp gauge while testing...don't allow engine to overheat.

SOURCE: temp gauge often reads too hot, engine normal.

It sounds like your cooling fans are not comming on by themselves, but when you turn on the ac it forces them to turn on. Make sure relay is good, then replace coolant temp sensor, this may have failed no telling pcm to turn on cooling fans. Thanks

SOURCE: help needed to figure out why truck says it over heating

you might want to check the wires going to the temperature sensor on the engine make sure it hasnt rubbed the protective coating off, this can give you a false reading, the safest way is to put a manual gauge in place of the temperature sensor and run engine to make sure the engine is not overheating. You can pick up a cheap manual temp gauge at your local parts store and install the temperature probe into the hole where the temp sensor was located and run the engine. It sounds like you have a guage malfunction. But is better to be safe than sorry.