You shouldnt be using a heat mat to heat a Geko tank! Heat Mats for Reptiles are for ambient warmth, that the Geko can goto when it needs, for eg at night when you have the Heat Light switched off
Heat Mats produce allot more heat when they are trapped between to hard surfaces, this is why they are good for Formicariums, as they can be pressed against the glass and controlled with a thermostat.
In a reptile tank, the heat mat should be inside the tank, on the base, covered with substrate, when used in this way, the heat does not stay trapped and so wont get as warm