How can I create 'gch.app' such that "open -a gch page.html" works like "open -a Google\ Chrome page.html"?
The application name is defined by the application itself, there is no easy way to use a different one for open. So in this case using an alias is probably the best option

alias gch='open -a Google\ Chrome'


Alternatively you could define Chrome as the default application for html files and just use open some_file.html.