I'm not convinced it is that game breaking. I mean I'm not going to sit here and say "GL does NOTHING for you!". But I don't see it as any stronger than something like Hanging Gardens. You get one free tech when techs are cheap. 9 out of 10 times, it is going to be used on Philosophy, and likely a slightly quicker NC. The strength of NC is scaling science in a high pop. capital, which Hanging Gardens also gives.
Honestly the best thing about the GL is probably the 1 GS point, 2 if you pair it with Oracle. Which means your first GS comes early enough where you could probably settle it as an academy and get good returns on it. Is this enough to say it is game breaking, that it decides the match alone? Eh... still not convinced. Taking the time to build both GL and Oracle is a massive time investment, and the player who focuses on strong infrastructure or other wonder combinations (Artemis + HG for example) is still going to be in a competitive position.
I will agree that in a OCC or maybe even just two cities, then the value of GL goes up. Partly because you are opting out of expansions anyway and partly because OCC gets more value out of conditional bonuses. What I mean is if you miss out on observatories in a high pop. 5 city empire all running Secularism GS's, you are still going to have decent science. In a OCC, not having that observatory is a huge hit since it is a large part of your total science. In the same way, getting earlier GS points and an earlier NC is huge for a OCC, since you can't make up that science through other means.