I know this is a really old post already, but for those who are just making the switch to GIMP 2.10.20, this is how to use the mouse wheel to zoom in / out and recenter the image at your cursor in GIMP:
1. Edit > Preferences > Input Devices > Input Controllers
2. Make sure "Main Mouse Wheel" is in list of Active Controllers, if not, select it in the left list and use the right arrow button to add it.
3. Double click on "Main Mouse Wheel" in list of Active Controllers
4. You will see a list of events. Click on "Scroll up (Ctrl)" and select "Edit Event".
5. Search for "zoom" and choose "Zoom In" (view-zoom-in), then press OK
6. Repeat steps 4-5 except now for Scroll Down (Zoom Out)
7. You may have to click "Clear Event" on anything that is conflicting with your new event, or on anything you don't want.
EDIT: If you had previously set custom events in GIMP 2.8, you may have to also use "Clear Event" on them here, since now the event names have actually changed.
8. You can use this same process to set scroll wheel shortcuts for brush radius size, etc.
9. Credit to TanUv90 on YouTube for this technique
10. EDIT: To set your tool size or brush size with the scroll wheel, use the process above but search for "increase"/ "decrease" and apply "Tool's Size Increase by 10" to "Scroll Up" and "Tool's Size Increase by 1" to "Scroll Up (Shift)", and the opposite events to Scroll Down / Scroll Down (Shift). Of course, that is just my personal preference, you can apply any of the available increase / decrease events to any of the available mouse wheel actions.