Her stats just don't hold up to Nearl.
Not really sure 60-70 Atk & Def diff is that big of a deal... esp. when Gummy comes with a nontrivial chance of critting for 150-200% demeji AND stunning the target. Also ~200 more HP for what it's worth but that seems sorta trivial.
The reason why Nearl is better is cause she can replace Saria in high risk CC runs while Gummy just isn't enough. Also they're both healing defenders so the important thing here who's better at being a healer...
I'll give you Gummy's S2 is definitely trickier to use properly, though that +50-80% Def during the first phase is nothing to scoff at (the buff is p much on par with eg. Cuora and Liskarms S1, though not the duration). I'd say their S1s are largely on par though given their slightly different details - given time Gummy will unconditionally heal anyone within range to full health while Nearl will sit there grooming her tail and giving no fucks about anyone over 50%. One is more general maintenance, the other more emergency personnel.
Nearl is ultimately the better healer, sure, given her talent and more convenient S2, but then she'd damn well better be because her kit's much more narrowly specialised. Gummy is more versatile and ofc Hell of a lot easier to get (and max out) in the first place.