Coolant temperature 75% towards hot (red zone)

Hi, this medium-mileage car has oil leaks that are coming from valve cover (hopefully not also from oil pan gasket and drain bolt washer). I am to replace that, but I recalled that the engine temperature gauge has always been higher than the middle point. I do not want to replace a gasket only to have it damaged or loosened by overheating.
Coolant level looks just fine between minimum and maximum (checked cold). However, even tho GM Dexcool is said to be good for 250,000 km or 150,000 miles or five years, I am not sure if it has been changed in awhile (seven to ten years). Should I be testing the coolant temperature sensor? Somehow, as-a-beginner-to-mechanics and car electrics, do resistance/voltage tests on wiring from ECU or instrument panel to verify if the temperature gauge on the cluster is even accurate? How do I really tell if the Pontiac's heat level is too high? I got Haynes manual but a lot of the photos/illustrations are dark and less clear. Thank you kindly.
I do have at least one round of experience doing a coolant flush without overheating on a Toyota Echo. Cannot under stand why GM car (Pontiac Sunfire) can apparently suffice with 'clean tap water' mixed in the coolant, according to owner's manual. Toyota strongly recommended distilled water.