I'm using a tool in QGIS where I'm creating points in a grid and I am specifying the points to be created every 0.5 of the user-defined atlas size. In other words, 0.5 map units of the CRS. What is the map unit of WGS 84? Is it km or m or degrees?
Be careful as WGS84 is a bit ambiguous.
Most of the time it mean EPSG: 4326 that is indeed in degrees (and the base of GPS position) but the name WGS84 could be used for other CRS with different unit (see image exemple below, taken from the website referenced in JGH answer). it's safer to use the EPSG code or to be sure to have the full CRS name to determine the unit.
It is in degrees. You can always check this repository for projection details