Buying somewhere else, using alternatives, such as green gas or heat pumps
...and then never go back
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.
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