Is there a beter way to check input besides a switch case?
You don't need a switch here.  

The easiest way is to just  

if(emptyValueIsDetected)
{
    ShowErrorMessage
    return;
}  


like  

if (GalleryName == null || GalleryName == "Default")
{
    Debug.Log("No Gallery name assigned, please give the gallery a proper name");
    return;
}  
if (gallerySize.x == 0 || gallerySize.y == 0 || gallerySize.width == 0 || gallerySize.height == 0)
{
    Debug.Log("No gallery Sizes assigned, please set some sizes before creating a gallery");
    return;
}  


and so on.

Naming  

The name indvImages for a variable is not really meaningful. If Mr.Maintainer will inherit this code, he will need some time to understand what the variable should mean.  


//Enable Bullet Buttons for the gallery
if (enbBB)  



If you need a comment to explain what a variable stands for, you haven't named your variable in a meaningful way. A comment should only describe why something has been done, and not what it is //This is a rectangle.