Looking briefly at the makefile, I think they might have avoided Gradle, though it calls other tools written in Java.
I'd love to see a way to build a Flutter app without Gradle.
unzip somewhere, set path variable
> yes | sdkmanager --licenses && sdkmanager "platform-tools" && sdkmanager "ndk-bundle" && sdkmanager "build-tools;33.0.0" "platforms;android-33"
Yes, you can just grab the zip files. For example like this: https://www.hanshq.net/command-line-android.html#sdk
You generally don't need to touch gradle at all with flutter. Is your issue about having to install gradle onto your machine, even if you don't use it directly?
(I used to work on it, or at least tried to before giving up in frustration.)
Out of interest, what blockers did you encounter using Flutter? For me it was much better than native android in terms of development, but it could depend on the usecase.