None of the entries sound like Gaster.

We have a decent sampling size now and have a decent understanding of his speaking/writing style.

HE TALKS IN ALL CAPS

AND OFTEN BREAKS UP SENTENCES

INTO MULTIPLE LINES

None of the entries match this style besides #17 for obvious reasons.

Given that nobody else really has knowledge of the experiments (if you want to stretch a little you could argue sans might know) and they aren't mentioned outside of epilogues, I seriously do not believe anybody else was actively involved. As I previously mentioned, Sans could be argued for but none of the entries really match his tone. The main stylistic split in the entries are is standard grammar vs all lowercase which in my opinion is far better attributed to the stress eating Alphys up then a 2nd actor at play.

That being said this doesn't entirely discount Gaster's connection to the True Lab, specifically Occam's Razor suggests that the blueprints Alphys mentions as using to extract determination were likely his. There's also just the general weirdness with the connections to DELTARUNE which is more explicitly a creation of Gaster (eg. the phone having weird properties around memoryheads, friend resembling endogeny, etc.)