Mgzr 120 heater problem

Hi I have just bought a mgzr 120 and on the way home which was a long drive the heater in the car didn't get that warm the switch worked well but even on full blast was not getting that hot . I also noticed that the temperature gauge was on half way untill a hill then it went up to just over three quarters just to the point was about to pull over it drop back down this repeated on every hill . When at home looked at the header tank water and it looks like someone has put k seal or something else in there , would this block heater matric ? Is it head gasket on the way out ?

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