Not a single reply about this in over a year? I find that quite surprising.
Here is what I know about this.
Nobody at Advanced Support for Partners has been able to address this or point us to any documents or other info regarding what qualifies a partner for AOS-G. I suspect it is two things: 1) you have been doing government license sales for a long time, and you sell a LOT, meaning upwards if half a million per year or more.
Back in early 2019, I was in touch with members of the One Commercial Partner team who shared that Microsoft intended to make all of GCC, including GCC High available via CSP. I was told at the time that Microsoft wanted to get the program details right. Those efforts clearly stalled, and by mid-year they had not only completely re-arranged the rules for CSP Direct, but had also made a great mess of GCC [Moderate] licenses that took most distributors several months to get their arms around.
I suspect somewhere along the way, those 6 or 7 businesses that are in this country club of sorts must have pushed back heavily on Microsoft to delay or scuttle the plans for opening GCC High to CSP. As of Feb 2020 at Ignite DC, I was unable to get reliable answers from anyone at Microsoft on this issue or the problems deploying Defender ATP into GCC tenants. Then COVID happened and communication with MS suffered a lot as a result.
Since then, we have become CSP direct and joined the CSP for GCC/Azure Govt. programs. If you have not done this yet, then I would suggest to start there. The process is messy and confusing, and there is a good chance it will take you working with MS for several months to get the kinks out of the system. I am sure if they ever open any GCC High licenses to CSP, they will do it through this program, because as a partner you will have to create a separate tenant in Azure Govt. / GCC High which will have its own access to Partner Center.
Microsoft has just pushed out a ton of licensing changes around Defender ATP that have the effect of re-aligning it from a component of Windows 10 Enterprise to a part of EMS. That may seem pointless, but in GCC [Moderate] it is the difference between the customer being able to purchase the better security tools vs. not being allowed to because MS365 G5 and Windows 10 subscriptions were not available through CSP.
Perhaps now that they have provided a fix for the problem that started in 2019, they will eventually work their way back to opening up higher-security tiers. In the meanwhile, I guess all any of us can do is to hold our breath and wait to see if anything changes. Find an AOS-G partner that you can partner with who will be responsive and not too overprotective of their relationship with your clients. Good luck!