gatsby --version > -bash: gatsby: command not found

you have to update your config. You may have to follow few steps

Set Config

remove existing config

npm config delete prefix 

set new config

npm config set prefix /usr/local

Install gatsby

npm i -g gatsby-cli

now check version

gatsby --version

Instead of trying to install globally, you can use npx

 npx gatsby new my_dir_name

Then run:

npx gatsby develop

Then:

npx gatsby build

And then you can run them using

npm run develop
npm run build