Ground squirrels mean different things in different parts of the country. Just so we're clear you mean what we call "chipmunks" right?
If yes, then a .22 is more than adequate. .177 would probably suffice, but you'll want pinpoint PCP accuracy so upgrading from your RWS springer would be a good plan. This is just my opinion though.
As far as which PCP you go with, two things I want to mention:
1) PCPs are incredibly addictive. Start shooting them, you'll probably keep shooting them. And this will scale up with time as you work out your fill solutions etc. I have both powder burners and airguns, and I shoot my airguns way more. They're just more fun, particularly the PCPs.
2) I generally recommend either buying absolute entry-level (like the gauntlet, or even cheaper) or going higher end with a "buy-once cry-once" philosophy where you buy the PCP you think you'll end up with rather than one you'll outgrow.
Just something to consider. There is a lot of love and some amount of hate for the Gauntlet, but there is no getting away from the fact that it is a lot of gun for the money. At the same time I wish, years ago, someone had told me to just suck it up and buy the expensive PCP to start, as it'd have saved me money in the long run. Something like an FX Dreamline and a Nomad II compressor can seem eye-wateringly expensive as a first dip in the PCP world, but it'll set you up with a good air source and a world-class gun you won't need to try and sell for an upgrade later. It will also give you the option of more power and other calibers should you want it to hunt other game.
My 2c. I hope it helps.
