The number of mythstones dropped by the Mini-Boss battle is listed along with the other multipliers when you select the difficulty of your Explore run.
I’ve always left GaP at Explore 1, which is a base of 3-6 mythstones. And I’ve usually done so in a kingdom with no 20% bonus active and at Kingdom Power Level 15 or below (because 16 grants another bonus).
Aye, either leave the mimic battle if you really want it by switching to another kingdom (although as @ALP mentioned this may not be worth doing as at E1 tokens are low) and coming back to it after GAP, or quit the battle to continue speeding through GAP. If you find the mythic mimic and need more copies, I recommend leaving it for later and switching to a different kingdom.
I’m not going to bother going back and forth with this OP, but for others reading - calling this a nerf to GAP is a huge stretch due to how slowly E1 battles fill your mythstones and the fact that you can leave the battle for later (if you care to fight the mimic) or run from the battle (if you would rather go through GAP in the current kingdom).
Looking forward to getting more medals and finally starting on some of those medalling kingdom power tasks I’ve been avoiding! 
Maybe for the folks that cannot wait to get the troop just have a pay option. $10 or $20 or whatever the Devs think is fair for most rare troop in game. The rest of us will get it when we get it. Not a big deal.
I don’t know if this is the way you play, but most of us start our GaP’s on low mythstones to ensure we are not interrupted by a Mythic troop with high health. This isn’t really an issue.
Thank you for your response
Wasnt a problem at all for me. Seems like a non issue
The rarity of being able to actually find Mimic battles makes this a non issue.
I ran 6 gaps last week. All on difficulty 9, never got a mimic battle.
So it’s like, are lotteries over taxed? Probably, but since it effects so very few people no one cares. 
The simple solution to this is to play enough level 12 runs before your GAP that you have fewer than 30 points towards your next Mythic battle. Then GAP at Explore 1 or Explore 2. Solved.
I do agree that the rarity of the cursed gnome makes the GAP appearances and reductions infuriating.
Heh; I had one GaP L7 run in which both of the two mythic chests I reached triggered the mimic battle.
(Clicked out and switched to another kingdom to continue the GaP; this costs a few extra clicks, but looping around back to the start of an explore is always pretty clicky anyway…)
This must be a joke coming from you after you had a long complaining thread on how mimic fights being always L150 would actually nerf the GaPs further. Not completely unexpected, but still …
It’s still a nerf. The impact of the nerf is too negligible for me to care longer than a week about it.
Let it go bro… Let it go…