Is there any tool for fine zoom in GIMP?
I mean not 125% or 100% , something in between.
Try holding in ctrl-key and scrolling up or down on the mouse-wheel. while doing this you can move your mouse-pointer in the desired area of the canvas you want to zoom in on, another way to more finely do this is hold ctrl-key in and hold in the scroll-wheel and move your mouse up or down on the canvas. this is probably what you are trying to do.
You can enter your own zoom ratio in the zoom indicator:
This is also doable via
Otherwise, with a given window size, you can use
View>Zoom>Fill window(your image is zoomed until its edges reach the edges of the window)
View>Zoom>Zoom to selection(your image is zoomed until the edges of the selection reach the edges of the window)
You can also click on the top right corner of your window, the zoom is then adjusted as the window size changes (the displayed image remains the same relative to the window):