Grid Maker
This Grid Maker program is a Beta version, currently available for Windows and Linux. You can easily import a map without a grid, overlay it with a square grid or hexgrid, color it, adjust the line thickness, and export it. It can be run without installation and can be carried on a USB drive.
The use of the program is completely free.
The program was created using the Godot Engine. If you have any feedback or suggestions, feel free to write on Kickstarter. (That's where you can reach me the fastest)