When it's saying 10% guard, it means its adding 10% of your 'max' guard. Which as szemyq has said, is (at base) 25% of your max HP counting con (but not +Hp items for some reason, might be a glitch). Thing is, some of these tooltips are... wrong? Or aren't giving us information we should have. Unbowed, for instance, says 10% but it seems to be more then 10%. Upgrade seems like it makes it do x2 the amount but... it already does a ton. Or at least, the proc Unbowed does - haven't tested it as a player skill.
Anyway thats the gist of it. 10% or 20% from say War Cry? That's 20% of your max guard, so 5 targets with War Cry and you have full guard (which is 25% of your max HP counting Con only). If you see a fade-touched material that says +3 or +5 guard on hit, it's talking exact numbers. It's literally +3 guard on hit, not 3%. So for a quick example if you have 600hp, that's 150 max guard. 10% of that would be 15, 20% would be 30. As you can imagine +guard is ultimately much less of a percentage, overall, but its on 'every' hit (not just attack). So certain abilities can let it build up 'extremely' fast since its not just base hits. DoT's don't count, least not per tick, just on apply.
That's the basics of it, other then that you got it right with guard being barrier but it takes all DR/% into account not just armor. 20% DR from the front on warrior, 50% halving range dmg all of that happens before armor. Defenses happen after armor. As such, something like the Ice Flask on Tempest-Rogue 85% dr would happen 'before' armor. Leaving only 15% dmg for armor to have to contend with. It's short lived, but a tempest ice flask rogue will often only take 1 dmg even from large hits due to that for its duration. Add some guard on hit and... yeah. Elemental Mine on Artificer, great way to build guard... each one (per target it hits) comes out to a lotta hits.