>>Buying somewhere else

Where? And how?

>>using alternatives, such as green gas

You mean synthetic gas? There is absolutely no capacity to produce anywhere near enough to satisfy even a single percent of European needs.

>>or heat pumps

Heat pumps are not a universal solution, they don't work in all climates, or they require digging trenches or wells to actually be efficient - again, not possible everywhere and for everyone.

And even if it was - with what money? Where I'm from in Poland I know lots of people who still heat their houses with coal, purely because gas is too expensive as it is. And you want to switch their source of heat to an even more expensive one? I'm sure they will be super happy to do that, sure.

Pretty sure there exists more than one source of gas.

You only need as much gas as is required to fill the gap that alternative gas providers and heat pumps can't provide to compensate the lost gas.

Another option is to improve the houses such they leak much less heat.

Money allocation could be managed through a variety of ways, such as EU funds

Heat pumps suck in really cold climates. I highly recommend a natural gas water heater if you have a gas line. I have never run out of hot water.