You can still run gscan locally without having a full local dev environment (as long as you have node & npm), by installing gscan on your machine:
npm install -g gscan@latest
And then running gscan in the folder where your theme files are:
gscan --fatal --verbose .
The --fatal flag only outputs errors that will stop Ghost accepting the theme, rather than general errors or warnings, and the --verbose flag outputs help info as well as the error.