Toughness is how much damage someone has to do to you in order to kill you (bullet damage, not exotic like fire).
It takes in to effect your armour mitigation and your health.
If I have 75% armour mitigation and 1,000 health then I would expect my toughness to be 4,000.
4,000 damage - 75% for armour = 1,000 - 1,000 health = death.
A lot of people go for Stamina on most of their equipment and then use mods for electronics / firearms (maybe one equip piece with firearms). Mods usually have a stat boost plus a bonus which can include, as mentioned above, armour, crit chance or skill power. Skill power is a good one as you can add 1k --> 1,5k Skill power per mod (depending on the mods level).
Blueprints for the mods can be found in the area before the Underground lift and the high level ones seem to rotate sometimes (I have lvl 32 firearms and tech from there) so you don't need to grind the DZ to get them now. Undergrounds will give you 1 gold crafting mat (if you can find it) for each phase. Crafting has become more relevant again now if you grind the UG a bit.
Test stuff out on the shooting range in the BoO or just on random mobs. HVT daily missions can also be a good place to test as on failure you don't loose anything and can just restart them without any extra intel spend.
Also remember that just because it has a higher GS, it may not be better and this is true for weapons and equipment. I still have a 163 M1A in my stash which is very good.