Are all the Green Lanterns together really only Galaxy level? Feat here comes from Green Lantern #25.
Let's take a look.
First thing to look into
is the statement for it being Galactic. This is legit, but it's really questionable if this is the maximum range of it, or at least the maximum potency. Let's take a similar example with Monarch, after Superboy Prime breaches his armor, a Universe destroying explosion comes out, and knocks out the 2-C, possibly 2-A Prime. The wiki
reasonably treats this as a 2-C, possibly 2-A feat on Atom's page, showing the Wiki excepts the idea of explosions like these having potency greater than their range. Not only this, but something that envelops the Universe would also cover the Milky Way Galaxy, so while nothing states this to be higher, there's no real reason to say this is the limit to it either.
On top of this, let's look into the Lanterns involved. While there's a lot of big names there,
Hal and Kyle are off fighting Sinestro, so it's not like literally every Lantern was there. Not to mention, they're literally in the middle of a war against the Sinestro Corps.
Hal and Kyle even approach 0% energy, and the Lanterns that contain it are all seemingly scratched up. On top of this, many of the Green Lanterns
still had energy left and continued to fight Prime, so they clearly didn't use all of their energy to do this.
In conclusion, nothing actually caps the bomb at Galaxy level. Its range is never limited, and even if it was only Galactic, its potency could be higher. It's also important to take into consideration the state of the Lanterns performing this feat, as many of them were at low levels, or would at least want to conserve energy, which would be a good reason to have as many contribute as possible. Even with this, some big names like Hal and Kyle were absent from this, and many of the ones who were didn't exhaust all their energy.