Let's Play policy
Flare Engine is a stand-alone piece of software. Its license (GPL) is completely separate from any game content. Without content there's not really anything to record. So permission to record a game needs to come from the makers of each game.
I'm the primary creator of the Alpha Demo (the currently released content that usually comes bundled with the Engine). The game also contains Free art (CC-BY-SA) made by many other artists. So legally I can't speak for the rest of those artists when it comes to recording the game. You definitely are allowed to release your recording under the same license as the game - CC-BY-SA.
My personal opinion is that Let's Plays (and similar) are a form of criticism/review. I think it ought to be Fair Use, but I don't get to make the rules. As far as I'm able to grant this, you have my permission to release commercial videos of playing the Alpha Demo. It'll be the same for any games we officially release here with that fantasycore art set.
Releasing your Flare video CC-BY-SA
To be completely in the clear, Let's Plays and similar videos of Flare's Alpha Demo should be released as CC-BY-SA 3.0. What does this mean and how to do it?
CC-BY-SA 3.0 states that you can share and remix Flare Game content for any purpose, even commercially. You must credit the original creators (Attribution requirement) and you must release your transformation under the same license (Share-Alike requirement).
In your video description, I'd recommend saying something like this which fulfills the Share-Alike requirement and Attribution requirement. Links included!
What does it mean if you release your Let's Play as CC-BY-SA? Others can remix/transform your video and release it too, as long as their work is also CC-BY-SA. Put simply: we shared our game with you, so you should share your video with others. Practically speaking it doesn't really change anything else about your video. You're clear to make ad revenue etc. as usual.
I say you're clear, but check your own contracts. E.g. if you're under an Exclusive license clause (e.g. your boss or video network wants exclusive rights to your Let's Play video) then you can't "Share-Alike".
All my original content, plus CC-BY music by Yubatake. You have my permission to release commercial videos of this game under any license. I suggest attribution like this:
Press Any Key To Live
Press Any Key To Live - All my original content. You have my permission to release commercial videos of this game under any license. Suggested attribution: