Am I allowed using "New Gauge Half-Life Client"? BunnymodXT doesn't work for me. I found NGHL works perfectly for me.

https://wiki.sourceruns.org/wiki/New_Gauge_Half-Life_Client

I do own Half-Life 1, this can be checked here - https://steamcommunity.com/profiles/76561198420139628/games/?tab=all

Newly submitted runs using NGHL client on the leaderboards will not be permitted. If you are having performance issues, it is recommended to use the 2005 package uploaded in resources, and there are also additional measurements to help you have better performance in-game. BXT is required for all runs to be verified.

Muty, I was told here that Maxam was going to break your record in Half-Life 1.

Thank you! Seems like they fixed false-detect for Bunnymod XT, cause that was an issue with GoldSRC package, that's why I switched to NGHL

NGHL is allowed you just need to configure it in a certain way.