How can I change the GIMP font?

You need to copy the gtkrc file that comes with the GIMP source (in the top level directory) into $HOME/.gimp/gtkrc . As of 0.99.10, this should be recognized. You then go in and edit the default font style, the one that looks like this:
style "default" { font =
"-adobe-helvetica-medium-r-normal--*-100-*-*-*-*-*-*" }

I'm sure ther's a lot of clever stuff that can be done here, and I'll try to track it down soon, but in the meantime, just change that "100" to something larger, like "120" or "140". (The number is points= 10, so 100 is a 10 point font).

Obviously, you could stick in any font you have available.

Choose Tools - Options - LibreOffice - View

Scaling
Uses percentile scaling for font size in user interface elements, such as dialogs and icon labels.

The Scaling setting does not affect the font size of text in a document.