Ye so as I sad 
neither drag, nor baron triggered my 
maybe the size of the model would be to big ? duno ? I triggered it earlier on a tier 2 tower... 1-2 shots before I would be killed. and woes taking constant dmg from almost my max HP...
yey i broke it :) ? 
Well, just quickly checked... it should be impossible for dragon to trigger it in these circumstances; even if the %current hp damage it deals was %max hp damage that ignores your resistances. Baron is a bit more complicated... I don't know the intervals at which he uses his abilities. But with the resistances you have, again, I doubt that he can trigger the item.
Don't forget that he needs to take the 1800 damage within 5 seconds for the item to trigger.
You have to take the damage within 5 seconds.
I don't think a tank would receive enough damage quickly enough from Dragon or Baron to activate Sterak's Gage. While they deal a significant smount of damage over time, they don't exactly deal damage in bursts; they deal it over a significant amount of time.
The fountain deals pure damage (which is different than true damage). Since only the fountain laser deals pure damage and pure damage bypasses the effects of many abilities and items (i.e. It will kill people through Tryndamere's ult, Kayle's ult, Kindred's ult, etc), it's quite possible that Riot decided that Sterak's Gage shouldn't activate from pure damage.
