I decided to dabble with Unity recently. The first project I wanted to play required me to use tilesets and tilemaps. I ended up finding a free tileset and followed a few tutorials, but whatever I did I always ended up with weird space between my tiles. Here is how I ended up fixing the empty space between tiles in Unity.
While looking at the scene view, I could not see the empty space but when I was switching to the game view I saw the image on the right. Since I was spending most of my time in the scene view, it took a while before I found that things were broken.
Once I saw the issue, I played with a few settings, trying various things. Double checked my tiles, their size, the import flow. Everything looked right. The one thing I never modified was my tileset. I assumed that it was built correctly since the import showed the tiles correctly. In hindsight, I should have investigated the tileset earlier.
After quite a bit of digging, I figured that in some cases (depending on the camera, pixel rounding, and such), Unity will go and pick pixels a little bit outside of the tile, taking the pixel next to the tile from the tileset. In my case, that pixel was transparent, so I ended up with transparent pixels in my tilemap.
After reading a few proposed solutions, and trying them, the only one that worked for me was quite simple: add a bleed around my tiles. At that point, I was still exploring and was using only one tileset with a few tiles. This made manually editing the tileset easy enough.