How to fix tile gaps in Unity
Whenever you're using the Tile Palette in Unity to paint tiles, sometimes you'll notice each tile has a gap in-between each tiles. To fix this, select the main sprite texture you're using for the tilemap, then under Inspector, change the Pixels Per Unit by 0.10 smaller than the original. E.g. if your tile is 48 x 48 per unit, change it to 47.50 x 47.50, that should resolve the issue. :)
I find this works 100% of the time.