The Power Gem is one of the six Infinity Gems that can only be acquired by completing a certain challenge
The Power Gem grants the user constant superhuman strength as long as the user holds it, allowing him to withstand many hits.
Step 1: Defeat Thanos and let him live
Step 2: Go to Asgard
Step 3: Request that Heimdall teleports you to Ego the Living Planet
Step 4: ...