I'm trying to do something in this video here: https://www.youtube.com/watch?v=kt-fI-GKs24 but with GIMP or Inkscape. So, basically, if I convert the infinity symbol to a path, is there an option to put text IN a path, instead of along a path? Just something that looks like the video shown.
There's no text inside a path function in Inkscape or GIMP. There are ways to flow text in a rectangle or shape in Inkscape, but it won't work for what you want.
You can put text on a path in Inkscape. I wouldn't use GIMP for this. GIMP is a photo editor. Better to do this in vector software.
I wouldn't convert the infinity symbol to a path either - it's unlikely to work. Instead I'd draw a path over the symbol to place the text on, using the Bézier Tool (aka Pen Tool).