what are possible causes for the gas guage not to work? I didn't check anything like the fuses etc. yet. but just thought I could get some ideas what to look into.

Often times improperly installed gauge faces cause the gas gauge to get stuck. More likely though its the sending unit. NO fun to change.

Looks like you will be dropping the tank

I assume it is stuck at E. Common cause is the float fills with fluid. Resistance of sender is 15 ohms empty and 170 ohms full. If you unplug the tank electrical connector, the gauge will go to full. If it doesn't, there is something wrong with the gauge or power to it.