Excluding subfolder from the Yii project

Hello All.
I have a Yii project up and running on website.com and wondering if I can add a subfolder to website.com/gallery/ where will be isolated ( not related to Yii) files and folders like index.html, style.css etc?

Right now if I just create that folder in the root directory it is showing a standard Yii 404 message. I have a feeling that a specific folder needs to be excluded from Yii system?
Can someone please help which file need to be modified and with what line?

Thank you for your help in advance and sorry if this topc was already raised as I was unable to find it.