Depends what you want to do with those character sheets.

If you just want a text file that people can paste their character sheet into, this will work: https://foundryvtt.com/packages/taf/ If you want to actually build full custom character sheets, you probably want something more like this: https://foundryvtt.com/packages/universal-tabletop-system/ Or this: https://foundryvtt.com/packages/custom-system-builder/

There's also a general purpose PbTA system that's meant to be customised into different PbTA hacks: https://foundryvtt.com/packages/pbta/

I guess in the sense that you have to load some kind of system to play, there's a requirement to use some sort of plugin, so maybe that's a dealbreaker. But it's generally not an issue. While premium content for Foundry exists, it's mostly in the form of rulebooks and scenarios that have been ported into it by the people who make and sell those games. The actual systems are all free, with literally only one exception (Brambletrek, for some reason).

Foundry has so many game systems, even systems like custom system builder, which let you make your own.

If all you want is spreadsheets, why not use shared Google docs or something?

So you want to be able to stream Gimp and have a shared drive with your PC's sheets, it needs to be open source and with no limitations?

I'd just do gimp+Discord+google docs, but if you want it to be open source and all-in-one then go checkout Nextcloud. I think that's as free as you get, if even foundry is too limiting.