The reason for the Image Gallery was for characters with a lot of images. Godfree, for example did not have many appearances that we felt needed a new page. But then there are characteres with several images.
So the rule is: If they have more than 3 or 4 images, make a subpage for the character; otherwise just put the gallery under a "Image Gallery" heading.
We only have separate gallery pages for characters that have more than 5 images available for a gallery (because galleries for characters tend to get really big). For all other pages, we just add the galleries on the article itself, since it's not worth making separate gallery pages for them. For example, http://swordartonline.wikia.com/wiki/Divine_Dragon_Alliance#Gallery is how a gallery is done for a guild. If you wish to add guild images or images for characters that don't have more than five pictures (Godfree, for example), you can just make a gallery like that.
It really doesn't matter whether you use underscores or spaces when uploading images. The wiki will handle them the same way. Though it is preferred to space words one way or the other to keep the name less cluttered.
Just because one guild has a lot of pictures doesn't mean that all the others do. I haven't seen your images, so I'm not sure what kind of images you are planning to add to the guild gallery, but guild galleries should only have images of the guild as a whole or a group of guild members (e.g. http://swordartonline.wikia.com/wiki/File:KOB_members.png), not individual members. Individual member pictures should be used for the known member table and character galleries.