For context: without a GSLT, servers are hidden from the Internet server list. Please refer to the documentation here for details on setup.
In older versions of the game there were no requirements for visibility on any of the server lists. This was great for hosts as it reduced the barrier to entry, but was abused by bad actors to bypass moderation and spam inappropriate listings. Requiring a GSLT makes it harder for bots and fake accounts while still being accessible to most Steam players.