I adapted the Ravencoin build script - by replacing references to Ravencoin with references to Gapcoin. The resulting Windows64 binaries are here (and have previously been advertised as such):

https://github.com/gjhiggins/gapcoin-core/releases/download/v0.1rc-alpha/gapcoin-0.16.3-win64-setup.exe

https://github.com/gjhiggins/gapcoin-core/releases/download/v0.1rc-alpha/gapcoin-0.16.3-win64.zip

(I did see your HMAC PR for v0.9 but as you observe, it is an ill fit with the 0.16.3 codebase)

The 0.20 client will remain unreleased because the code in 0.16.3 that enables a soft-fork segwit transition has been removed in the Bitcoin 0.20 client. This is because the Bitcoin blockchain is now irrevocably segwit-enabled and the soft-fork code has been replaced by a simple height switch. Without this soft-fork transition code, upgrading to the 0.20 client would unfortunately necessitate a hard fork for the current Gapcoin chain AIUI and according to the chainz explorer, the overwhelming majority of Gapcoin nodes are Gapcoin 0.9.2 vintage, likely original Windows clients and there seems to be little likelihood of that changing.

Cheers

Graham