Question #1: how are your keyboard chops? 49 keys is a lot
better for two handed playing than 37 keys.
Question #2: do you own any other hardware synths? Gaia has very "hands on" programming. Even with the knobs of the MPK, working with softsynths is never as immediate. Gaia is a very basic instrument, great for learning subtractive synth programming if you are more of a preset user looking to learn how to create your own sounds. The actual sound of the Gaia is good but not mindblowing (not to me anyway!).
Question #3: how much need do you have for the controller features of the MPK (pads, sliders, knobs)? You can use Gaia's sliders and knobs to control software but it's clumsy compared to the MPK. Compared to the knobless Keystation, even the Gaia is a step up in terms of softsynth control
Question #4: do you already own an audio interface for your computer? The Gaia can be used as a USB audio/MIDI interface.
Question #4: what's your DAW? MPK comes with templates to control most of the big name DAWs and I believe it ships with a copy of Ableton Live Lite.
I listened to Hatfield and the North at Rainbow. They were very wonderful and they made my heart a prisoner.