Try this way
If the fonts dock is not there open it Gimp menu Windows -> Dockable Dialogues -> Fonts
Make sure the Font tool is selected and the Font is active (click in the text)
Select a font from the fonts dock, use the keyboard arrow keys to scroll through the list.
example https://i.imgur.com/AQ67Gxu.mp4
Edit: Sorry That quickly gives a change of font for the whole text layer but as Ofnuts stated not selected text.
If the fonts dock is not there open it Gimp menu Windows -> Dockable Dialogues -> Fonts
Make sure the Font tool is selected and the Font is active (click in the text)
Select a font from the fonts dock, use the keyboard arrow keys to scroll through the list.
example https://i.imgur.com/AQ67Gxu.mp4
Edit: Sorry That quickly gives a change of font for the whole text layer but as Ofnuts stated not selected text.