It is going to be an uphill journey for you to learn Gimp. Forget how "All other graphics programs" work
As noted earlier, your screenshot shows a grid-line. You can set up on a global basis
Edit -> Preferences -> Default Image -> Default Grid Also there in
Edit -> Preferences -> Image Windows -> Snapping is a snapping distance, default is quite small 8 pix. It can catch you out. Adjust to suit.
Working on an image, if for any reason you find that the grid is not convenient, colours, spacing etc it can be changed per image
Image -> Guides -> Grid (1) . Same place you can set up a guide, blue line that extends outside the canvas. (2).
Grid / Guide lines are virtual, have no physical size. These settings are saved in a Gimp .xcf image.
Grid / Guides / Snapping is toggled On / Off in the View menu.
Thanks yes i understand the guides and manual entry, the grid size i need to look at and see what its set at, i rarely use snap to anything. Good to know about the 8 pixels snap. The images i am working on and resizing already have their size? i mean pixels, so im not creating images with paint etc. I have to deal with resizing the best i can, and never thought about the line cutting through the center of a pixel instead of on its edges. Yes uphill battle, frankly gimp is not my cup of tea, so to speak, I am thankful its available, and of course free.
If you are unsure that the crop is exactly as you need it, use the Crop Tool Options and manually enter Position / Size as required.
Attachment:
grid.jpg