As a marauder I like to use Ground Slam, and would like to increase the damage with Iron Will. But when I equip the gem it does not increase the listed damage of the skill. While other support gems like Added Fire Damage and Melee Physical Damage do directly increase the listed damage.
Is there some sort of hidden increase in damage?
Am I misunderstanding the skill here?
Should Iron Will increase the listed damage if it works?
Look at the tags for the gems. Iron Will has the Spell tag. Ground Slam has the Attack tag. So Iron Will does not work with Ground Slam. Also note that if the support gem does work with the skill gem, you'll see a letter for that support gem on the icon for that skill gem in your hotbar.