I fucking hate this word but it's applicable here - TTGL is easily the most overrated anime of all time, beating out even Bebop easily.
I rewatched G Gundam a few months ago, and that was far better. I like TTGL but the reverence and worship it gets is baffling.
Probably because TTGL is just one of those shows that meets and exceeds expectations for the final stretch, when so many series bungle it at the end. (Hi, Kill la Kill.)
A lot of the series is in the end ultimately forgettable but there are a handful of really good episodes sprinkled throughout, even during the bad parts. Episodes 7 and 8, 11, 14 and 15, I really like the prison episode because of Simon and Viral's dynamic there, the Yomako episode is an enjoyable break from Rossiu's bullshit, and then almost everything there on is amazing.
I'd say there's a lot more "overrated" shows by the general public to complain about like Attack on Titan, Fairy Tail, etc. that have a lot less redeeming qualities than Gurren Lagann does.
I mean, there's probably still a lot of people who like anime and either don't know of it or haven't watched it. The most places you're going to get recommendations for Gurren Lagann will be on enthusiast boards, much like how gaming side has a Platinum Games boner but most people don't even know who Platinum is let alone why Platinum exists.