If you just join Mysterea first, pick up refreshment and a mana point and switch to GG, you're refreshing your mana and working towards Enlightment which is awesome for elves. And as Darvin said - thats just giving gnomes 3 more full mana pools on top of the potions and crystall ball. Pick a spellcasting Sorcerer or a Bloodmage who can convert B2P after he's done with blackspace, stick a mage plate on them for good measure, and it gets silly.
Cost is too low atm, I think (just because it requires not a lot of dedication to get to 50/60 + 50 to conv out as Mystera and it IS a very good boon). Extra glyph is a no-no as an intensive spellcaster because you want fireball from the getgo (unless you're a wizard, ofc), similarly prepped Mystera for caster assassins and wizards etc. means you tend not to get an extra glyph from worshipping another piety farm first. Getting Mage Plate via quest items is too unreliable for my liking, really. Synergy with GG is unreliable... the really big spike ofc is the Mystera->EM conversion