UnityPixelArt is a command-line tool to fix gaps between objects by adding redudant edges to all tiles of a spritesheet.
Insert the following line into your PATH system variable (Google how to do it if you're unsure)
LOCATION_OF_FOLDER\UnityPixelArt.App\bin\Release\netcoreapp3.0\publish
From the command-line you can now type PixelFix {FileName} (Include extension, ".png").
The command line will now start asking for data, all units are in pixels
"Input X Tile Size" - The Horizontal size of every tile.
"Input Y Tile Size" - The Vertical size of every tile.
"Input X Offset" - The X Offset from the top left corner.
"Input Y Offset" - The Y Offset from the top left corner.
"Input X Padding" - The X Padding between each tile.
"Input Y Padding" - The Y Padding between each tile.
This will output a file (filename suffixed with _modified.png) with redundant edges to fix the pixel-gaps.
Progress bar by using async method calls while modifying each tile.