It sounds like you are trying to modify production code in your live production environment. What happens when you do something to break the site? How long can that site afford to be off-line, or have things not working quite right? It is my experience that new development should be done in a development environment and then promoted to production. Otherwise you are forced to test your changes in your live production environment, and that is a dangerous thing indeed.
Given that you are new to Yii, odds are that you aren't going to be able to write code that is bug free the first time.
Best thing to do is create a development environment to mirror production, and install Gii there. Then you can create all the models and controllers you want with Gii, test your changes, and finally promote your changes to the production environment. This is really a best practice issue. You don't want to be changing your live code with untested code.