Log an error encountered when compiling google gn "I could not find a ".gn" file ..."

Applying Google gn+nijia, when compiling the program on ubuntu7.9, I encountered such an error message

# gn gen out/

"ERROR Can't find source root.
I could not find a ".gn" file in the current directory or any parent,
and the --root command-line argument was not specified."

According to the prompt, the .gn file should be missing in the current directory.Keyword search, someone has encountered similar problems on stackoverflow

selenium chromedriver - How to build with targets with Google Chromium GN? - Stack Overflow

The solution is as follows

I tried following the steps, but it didn't work.

Then go to the gn demo program and have a look, the ".gn" file is indeed in the demo program (note that the ".gn" file cannot be viewed with the ls command, and the ll -a command is required to view it)

.gn file content is

# The location of the build configuration file.buildconfig = "//build/BUILDCONFIG.gn"

Copy the ".gn" file to the current directory of the project, the same directory as "BUILD.gn", and then use the command

gn gen out/

Recompile the program, the error message "I could not find a ".gn" file ..." is gone.