Scrapped a rather long general post - back to the actual question.
No specific tool that I know, maybe someone knows of/will write a script.
For Gimp 2.10 a workaround other than some arithmetic to determine the center.
Use the unified transform tool on the canvas. looks like this https://i.imgur.com/8oHCcg6.jpg
Pull the edges in to snap on the existing guides: https://i.imgur.com/CPAX4iq.jpg
The default center is marked, zoom in, drag in a guide: https://i.imgur.com/rV8qE9S.jpg
Close the tool without using it, ESC or close the on screen dialogue.
if you don't like that tool, same thing but using rectangular selection tool, options for center / thirds ... https://i.imgur.com/tYUHAgf.jpg
Quote:..In Photoshop, you can place a guide in the exact center of two existing guides. How do I do that in gimp?
No specific tool that I know, maybe someone knows of/will write a script.
For Gimp 2.10 a workaround other than some arithmetic to determine the center.
Use the unified transform tool on the canvas. looks like this https://i.imgur.com/8oHCcg6.jpg
Pull the edges in to snap on the existing guides: https://i.imgur.com/CPAX4iq.jpg
The default center is marked, zoom in, drag in a guide: https://i.imgur.com/rV8qE9S.jpg
Close the tool without using it, ESC or close the on screen dialogue.
if you don't like that tool, same thing but using rectangular selection tool, options for center / thirds ... https://i.imgur.com/tYUHAgf.jpg