The contents are more mundane than supernatural. Many words are spent to remind the reader of druidic spells, nature spirits, outdoor monsters, etc., but these appear almost entirely in the fluff text. The crunch text describes what happens to people who
don't have fantastic abilities, and would suit any game where the PCs are limited to mundane abilities and TL0-4 gear.
The contents are also more cinematic than realistic. Chronic outdoor hazards (like cold and sunburn) serve to chip away at resources – FP, HP, rations, bug juice, etc. – so that unprepared travelers are somewhat weakened. These are not so much primary dangers as they are force multipliers for active threats like animals and highwaymen, who attack fresh rather than used-up. As in the dungeon, killing the PCs outright is saved for suitably dramatic
acute hazards, which happen to be wilderness-themed: lightning, tornados, deadfall traps, etc. This approach is genre- and TL-neutral, but better suited to larger-than-life campaign styles than to gritty ones.
You could use most of this stuff with, say,
GURPS Action or
GURPS Tales of the Solar Patrol without adjusting for anything but gear TL. You would not want to use this in a gritty
GURPS SEALs in Vietnam campaign or
GURPS Reign of Steel game. If your specific interest is post-apoc, I'd put the realism level at that seen in the
Fallout games, which are superficially kind of tough because most first-person shooters don't care about food and weather at all, but in fact very generous when you look at the timescales and actual effects.