I do think that in term of raw power gajeel is stronger than erza , but In other aspects I think they're pretty much the same. Erza durability is impressive and definitely above gajeel ( despite the bs of the fight with demon , it's a facts that erza have higher durability )
I'm not that impress with erza nakagami armor anymore , we have seen she use the armor In this arc and it's still not strong enough to defeat ajeel ( if she can use it much more longer instead of just a few second I might puts her above gajeel )
Erza plot armor (nakagami) is proven can't one shot everyone and she can use it just for a few second . While gajeel In his base form is already impressive . His other form is beyond impressive .( awesome maybe?

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I think they're both pretty much equal if we combine their durability strength whatsoever . I would puts erza the same level as gajeel .