An image gallery that does what? I'm not sure what you're asking the gallery to do. However, I'm sure there are tons of image galleries out there that you could use other than CB.
what image gallery would that be in particular? Sometimes, it would make more sense for use to create a special "type" for contentbuilder that connects to this gallery as this gallery might have its own way on image manipulation upon uploads. That would still make use of a contentbuilder based form but then specials can be built-in that only refer to the image gallery.
If you take the breezingforms integration for example, it is nothing else than an interface between breezingforms and contentbuilder (1 file and a few modifications within breezingforms to make it better integrating to use native bf forms, but that isnt probably necessary for your image gallery).