Why so many versions, settings, and developers?

This is not the typical app you find in the Play Store. There are no "universal settings", some phones support all features while others don't, it's stable on some phones but crashes on others, and it's updated regularly.

To make your life easier, I suggest some versions, but this port is hard and I can't say for sure that they are the best versions for your phone. The same is true when it comes to settings/configuration.

You should find other people using the same phone as you and see which port/version/settings they're using. See if there's a GCam thread for your phone on XDA or posts on your phone's subreddit (see: useful links and files). Discuss your issues, try to find stable versions and see which dev/version/settings are better for you.

Why so many updates?

First of all, if the version you're using works fine, there's no need to try all new versions. If something is broken or if you want to try new features, then yes, keep testing new versions.

In order to improve photo quality, unlock new features, fix bugs on their phones, supporting new ones, etc, developers have to share their apks. Users are then able to test and provide feedback. Some of these versions work, some don't. Some are better than others, and that's why it's important to be part of a community where everyone shares their feedback.

I upload everything to the download page because I can't test new versions on all phones. Sometimes a test version is a failure for a specific phone, but works very well on other phones.

Differences between developers:

You can find a description about each developer and links to their download page here: app developers and download pages.