TL;DR - you're probably out of luck....
I downloaded ELECTsUM on my phone thinking it was different than what it was.
To anyone else reading this: ELECTsUM is apparently a watching-only Electrum 1.x compatible wallet for iOS; I don't own any iOS devices and have never tried it.
For the master public key I entered the public key that is generated by my GPG Keychain
I'm very confused. I can't think of any way GPG could export anything that looks remotely like an Electrum 1.x master public key, so I can't figure out how you managed to do this....
It then generated 15 wallets
I assume you mean addresses (not that this matters)?
the 'next' button [inside the real Electrum client] remains gray and I cannot click it?
Whatever it is you're pasting in, if you got it from GPG, it's surely not a valid Electrum master public key -- that's why the button remains gray. If ELECTsUM accepted what you pasted in as a master public key, then that sounds like a (pretty egregious) bug in ELECTsUM.
Can someone please lend a helping hand here?
Regardless of all of the above, if you didn't get a master public key from the official Electrum client, any funds you sent to the addresses displayed inside ELECTsUM are likely lost forever
Edited to add: and if ELECTsUM accepts as input something that isn't at least in the right format of an Electrum 1.x master public key (128 hex digits), then shame on ELECTsUM!