What changes for you? Nothing unless you start using threads. You probably weren't using threads anyway because there is little to no point in python to using them. Most python code bases completely ignore the threading module and instead use non blocking IO, async, or similar things. The GIL thing only kicks in if you actually use threads.
If you don't use threads, removing the GIL changes nothing. There's no code that will break. All those C libraries that aren't thread safe are still single threaded, etc. Only if you now start using threads do you need to pay attention.
There's some threaded python code of course that people may have written in python somewhat naively in the hope that it would make things faster that is constantly hitting the GIL and is effectively single threaded. That code now might run a little faster. And probably with more bugs because naive threaded code tends to have those.
But a simple solution to address your fears: simply don't use threads. You'll be fine.
Or learn how to use threads. Because now you finally can and it isn't that hard if you have the right abstractions. I'm sure those will follow in future releases. Structured concurrency is probably high on the agenda of some people in the community.
Coming from the Java world, you don't know what you're missing. Looking inside an application and seeing a bunch of threadpools managed by competing frameworks, debugging timeouts and discovering that tasks are waiting more than a second to get scheduled on the wrong threadpool, tearing your hair out because someone split a tiny sub-10μs bit of computation into two tasks and scheduling the second takes a hundred times longer than the actual work done, adding a library for a trivial bit of functionality and discovering that it spins up yet another threadpool when you initialize it.
(I'm mostly being tongue in cheek here because I know it's nice to have threading when you need it.)