I checked as many the Age of Sigmar books as I could acquire, and I can’t find any reference to Gotrek or Felix by name.
However, we do have a bit of likely information about Gotrek. At the end of the End Times, as noted in the question, Gotrek took on the role of Grimnir. As such, it’s likely that any references to Grimnir in Age of Sigmar works refer, in reality, to Gotrek. According to the sourcebook transcript here:
While exploring the Realm of Chamon Sigmar found the Duardin Gods
Grungni and Grimnir imprisoned at the top of the Iron Mountains. How
they came to be there, they would not say, but each readily swore the
oath of allegience and set about repaying their debt to Sigmar.
Grungni, now crippled in form and unable to raise his hammer and fight
alongside his brother-god, promised to fabricate whatever Sigmar
desired as his smithing skills were unharmed. Grungni gathered the
remaining Duardin and established the Iron Karak, a bastion of the
Steamhead Duardin that would always answer Sigmars call. However
Grimnir was a warrior and asked Sigmar to name an enemy worthy of his
axes and that it would die, Sigmar named Vulcatrix the Mother of
Salamanders that terrorized the Realm of Aqshy. Though the battle was
titanic and created the Plains of Aqshy by its very occurence, the
end result was that both the Slayer God and the Ur-Salamander
destroyed each other. Where the hot coal remnants of Vulcatrix landed
a new volcano was born, but where Grimnir’s fragments ended up is a
mystery that the Duardian share with no one.
Those fragments of Grimnir/Gotrek are ur-gold, which the duardin (dwarves) seek to reforge someday into Grimnir.
‘Ur-gold is what Grimnir left us of his power and will,’ said Rolk,
staring into the complex geometry of the rune in his palm, his hard
face appearing to express something like veneration in the shifting
light of the flame. ‘By harnessing its might we do him glory in the
manner in which he approves.’
Age of Sigmar: Warbeast
Of course, there remains the possibility that the Grimnir referred to here is the original Grimnir, not Gotrek-as-Grimnir. If that is the case, we don’t know anything about Gortrek’s fate, save perhaps that he continues to fight eternally in the Realm of Chaos.