Is there some way to use GNU Privacy Guard functionality directly? Yes, I know about redirecting input/output streams from gpg.exe utility, but maybe we can use some unmanaged libraries from distribution directly?
I think I can answer my own question. I was pointed to Starksoft .NET GnuPG Component - a free library that takes interaction with GPG on itself. Looks like it works fine, even if the project seems to be not maintained for a while.
UPD: The component seems to be available now as a NuGet package at https://www.nuget.org/packages/starksoft.aspen