Venezuelasilverado

Dear all thanks for letting me in here . I have an issue with my 2011 silverado 6.2
It started overheating when driving mountains i used to live on isla magarita where the landscape is very flat , so the problem may have been going on for a while un noticed , as during regular driving there are no issues
The car is produced here in Venezuela , so the temp gauge is in celsius when the needle point straight up , the temp gauge says 100 degrees aka boiling point of unpressurized water , with no coolant fluid . Its the logic position for the needle , (straight up ) but a bit high if you ask me ? So during high temp climate steep uphill driving , it will throw water out of the expansion tank and it will boil , temp increasing to max , obviously i stop , let it cool down and add water . So far no blown head gasket . I wanted to take out and check the thermostat , but it has all ready been taken out by previous owner so no help there ! My questions are following
The water seems to go out through a valve/ hole in the bottom of expansion tank , and it will loose water in a way so that the system is unpressurized, is this normal ? There is a pressure cap on exp tank but since the valve in the bottom opens it has no function ? Is the temp normal ? Has anyone had a similar problem and how did they solve it ?