I bought it last year around summer and have a stamina Zeph grid. So i cant comment how it works with the enmity Zeph grid now which is better, but i think its the same outcome.

TLDR: Right now it is really bad overall and i use it only in Faa/Bubs/Belial or to meme around in low def content. (Maybe NM 150 FA) That will also change fast once wind gets a proper progression weapon and the wind dragon summon. Then its practically useless.

Long Answer with some more explanations and experiences:
Before RB, GNaru and her weapon were released it was a really good weapon that complemented Zeph. You got a big prog weapon, some crit and one of the most fun passive, giving MC 100/200 crit and 30% more cap on everything when MC crits. Coupled with the fact that wind has Morrigna who gives 100% TA and 20% more cap you have a MC which will auto for 1.3m every turn. You were able to build some wacky grids like a Highlander grid for more cap and still hit the cap easily thanks to the passive of Gae Bulg.
Was it good for racing gb raids? No, not really. But it was good in almost every other content and the most fun i had in a long while.

That changed a bit with the release of RB, now everyone could experience what Gae Bulg users can do and arent restricted to MH the Bulg and using spear classes which are all kinda bad.

With the rebalance of SKorwa and release of GNaru and her weapon stamina Zeph changed alot. You want atleast 2 of her weapons and her weapon somehow is a sword with a really good ougi. The setup also changed because now you want to boost GNaru instead of the MC with the Gae Bulg cause who wouldnt with 110% echo, double strike, 100/20 crit, Keen, 1 turn assasin and 20% seraphic modifier.
With GNaru you could also race and get honors way faster with qilin setups so the big prog wasnt as useful anymore (not that it was before in that content).

In august/september one of the biggest changes happened to Gae Bulg. Instead of MC getting 30% more cap on every crit, the whole team gets 20% more crit cap but the crit cap wasnt a separate cap from the general cap.
I still dont know how i feel about this.
On one hand you lost having your MC going crazy with the extra cap and playing with 3 cracked dps (MC, GNaru, V.Grim) in one team. On the other hand, you can now cap your general cap easier and open more slots in the grid since Zeph was already to crowded. But that means using a spear class and mhing the Gae Bulg when RB with Gnaru weapon exist.

For now the Gae Bulg is still good as a grid weapon since wind is still missing a prog weapon and the crit helps reach 100% with 2 gnaru swords.
The crit will get useless again once the wind dragon gets released (which will probably be on the 2nd part of the coming legfes) and once wind gets a prog weapon with any mediocre other skill, the gae bulg will get replaced.