It sounds as if the fuel has emulsified or the volatile elements have evaporated. Do you know how much was in the tank before you put the new lot in?
If the tank itself is in good condition, the only way to save it is to empty it completely and clean it out. Bilt Hamber offer a product that might be suitable:
https://www.bilthamber.com/fuel-tank-cleaning-kit
Do not try to run the engine - the likelihood is that you'll clog up the filters etc.