I don't think my SimBrief has actually ever worked with GSX (not surprised)
I don't know what you mean by that "not surprised" comment. Simbrief surely works with GSX, and if it doesn't (or it appears not to), there's always a reason.
but after trying all solutions I have found online I decided to give it a go here.
This should have been the first place to test. The only other time you posted here, you got a fairly thorough reply, which you never followed on with:
https://www.fsdreamteam.com/forum/index.php/topic,31094.msg198832.html#msg198832I can tell you that the numbers (pax/payload etc.) never match between simbrief and gsx.
Exactly like the other time, it's impossible to help you with such a sparse description of the problem.
For example, you don't say anything about which airplane you are using because, one thing is saying "Simbrief doesn't work", suggesting any number of possible issues like wrong username, flightplane not accepted, firewall/antivirus issues, connection issues or anything that would prevent Simbrief to even be used, to something like this, which indicates Simbrief appears to be loaded, but the results are not what you are expecting.
Since this is highly depending on the airplane used and how it's loaded, it would be a good idea to start with that.
And, GSX has a logging feature which you can enable in the Troubleshooting section of the Settings page ( Page 34 of the Manual ) so, by providing a log made with an example flightplan, and Attach it here as a file attachment (ZIP it first, so the forum will accept it), it will be very easy for us to understand exactly what's going on.