Scarlet Witch is still the most powerful being in the MCU. A fight between her and Gai'a would be so one-sided it's not even funny
Also, worth noting that Gai'a is missing tech and magic-based powers. She's also missing Peter Parker's DNA, and the DNA of post-Endgame heroes (like Spectrum). So it's far from a complete set of powers. Most of what she gained was just variants on super strength and durability, but there are some standouts:
Thor Odinson (Lightning)
Carol Danvers (Energy blasts & Flight)
Ghost (Phasing)
Mantis (Empathy)
Ebony Maw (Telekenisis)
Frost Giant (Ability to form frost weapons and frost armour)
These powers make her a huge threat & probably the second most powerful MCU character at the moment (after Scarlet Witch). But I wouldn't say she's unstoppable. She killed Gravik with Carol Danvers' powers, proving that she herself isn't invulnerable. I doubt anyone other than Wanda could beat her solo, but it's not difficult to imagine a team who would stand a good chance.