Gateway is the company that had the first public method of piracy on the 3DS, and the very first 'CFW' (3DS CFW isn't exactly custom firmware, but more like a series of patches that enable piracy.). They were the first to have a few exploits that have become important today, like firmlaunchhax, and memchunkhax, as well as the first ones to have things like an actual implementation of the MSET bug documented by ichtfly, their Spider browser exploit, MSET Downgrading on N3DS and regular downgrading. emuNAND. These were later reverse-engineered and made public by popular members of the scene.
Stuff like rxTools and Pasta were closely (and loosely) based on Gateway's exploits.
As of late there's no use in having the cart due to free open source CFW becoming just as capable as them, and people hate them as a company because they bricked companies that tried to clone their work and sell it. This once affected legitimate Gateway owners, but ever since it's made its return, there haven't been any reports of this.
tl;dr, they were the leaders of the scene for a time with their variety of exploits and later became obsolete after those exploits were reverse engineered and documented to the public. CFW has come far and sort of left Gateway behind, and due to some of the actions made by the company, (extremely long wait for N3DS support, brick code) they have been looked down upon by the community and have become sort of a meme.