Strictly speaking you can use any noun as a verb in English. The grammar allows for it, though most nouns will not have a meaningful verb counterpart, semantically, or it will not have a usage that most people will understand (i.e. it won't be colloquial)

So in a speed running context, I would definitely say it is acceptable, but it might not be in the greater population. But the grammar nazi have nothing on this one.

Yes it is being used as a verb. It's at least common terminology among speedrunners.

Many games, including the one in question, have an in-game progress tracker. To "100%" the game is to complete everything necessary to make it reach 100%.

For games without such a tracker, the community usually reaches a consensus on what is considered to be 100%.

If it isn't, I'm invoking my right to English incorrectly, given that it's not my native language ;)

(yes, I'm being toung-in-cheek by using "English" as a verb here :) )

There's even a relevant Calvin and Hobbes comic:

http://www.gocomics.com/calvinandhobbes/1993/01/25