Photoreal scenery isn't normally created with gmax, as it's used for modeling. Photoreal is created with aerial imagery obtained from programs such as SBuilderX 3.15 or SASPlanet. Governments also sometimes provide this data as GeoTiffs, which can be further combined into bigger working areas with GlobalMapper (payware) or Qgis (freeware). A global source that can sometimes have decent imagery available is
USGS EarthExplorer.
Here you can find two tutorials on photoreal creation. The first is an older tutorial, but based on the tutorial I learned from:
As far as mesh creation goes, you will need to download DEM data from a resource such as Earth Explorer or another source. I know Canada has a nice 19m data set available for download. I do not have a link handy, but a google search should turn up a result on this.
I made a tutorial on creating terrain mesh using GlobalMapper, however the same result can also be made using Qgis. The interface is just different.