ArmA 3 terrain builder uses UTM zone 31N with the odd easting 200,000 and northing 0 values.

If you are downloading real world data from some other UTM zone than 31N, then you must use that locations zone instead, otherwise the data is all stretched to almost beyond recognition.

For example Iraq WGS 84 UTM zone is 38N instead of 31N. If you are downloading data from Iraq, use Global Mapper / QGIS WGS 84 UTM zone 38N. And if you are downloading data from Afghanistan, use WGS 84 / UTM zone 42N, etc.

Check whole globes zones from DMAP: UTM Grid Zones of the World page. Also you can search “UTM zones <countryname>” with google, for example time of writing we found georepository for Iraq.

Excellent interactive zoom-able google/maps for UTM Zones, highly recommended.