I'm not completely sure from the error you describe but it sounds like you don't have a .gn file in the root.
I recently started getting into gn and made a simple starter project: https://github.com/aellerton/experiment-learn-gn
The first thing I didn't expect was the need to have a root .gn file, and that the file needs to point to the root config: https://github.com/aellerton/experiment-learn-gn/blob/master/.gn
The second thing I didn't expect was the need to provide an entire toolchain. I kind of like this now as it gives me full power. I used this one here, by making it a git submodule as the author suggests: https://github.com/timniederhausen/gn-build
Then building a trivial Hello World console program was straightforward.