I'm relatively new to GIMP and now having the following problem. As of today, my tool options tab is only showing GEGL-editing. Out of everthing I've tried so far (including reinstalling GIMP) nothing seems to help. I'd really like my old tab (with opacity, size etc) back! Any ideas on how to solve this?
Reinstalling the software won't fix this. What you're describing is normal behaviour. There's nothing wrong.
After you've used a GEGL filter* and applied it, and before you continue editing, choose another Tool in the toolbox, such as the Paint Brush or the Move Tool, etc, and the tool options will revert to normal for the selected tool.
*Note: GEGL filters and operations appear in GIMP's menus with a large letter G icon before them.