tl;dr: This is not the typical app you find in the Play Store. It needs to be configured, there are no "universal settings", some phones support all features and 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. Discuss your issues, try to find stable versions and see which dev/version/settings are better for you. I've compiled a few links, check the Useful Files and Links page.
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 device, but works very well on different device.
Differences between developers:
BSG, or B-S-G, is the main developer working on this port. Most apks from other devs are based on his work. He owns a Xiaomi Mi 5S and a Xiaomi Mi6 and shares his apks on 4PDA, a russian forum for mobile stuff. BSG's apks will work better on the Mi5S/Mi6 and some times the settings/features included are not compatible with other phones.
Arnova8G2 owns the Xiaomi Mi 5, OP5T, and OP6. On top of porting new features and trying different things to improve photo quality, he makes BSG's work compatible with more phones (other Xiaomi devices, OnePlus, Samsung, etc). His versions usually come with better/easier to understand settings/menus and work better on phones that need extra fixes (eg: OP3/3T/5/5T).
UriKill usually creates versions with lot's of options. Might be a good option for those who want more features.
cstark27 makes small modifications to apks released by other developers in order to optimise them for his LG V30. Some of his versions also work on Samsung, Nokia, Essential, etc, phones and dual camera devices.
Usually based on APKs from Arnova8G2, he does minimal changes to the menus, disables unstable settings, etc.
Tolyan009 makes small changes to BSG's apks in order to improve stability and adds some features.
Scrubber's versions are made to work on his Xiaomi Mi Max 2. They are very minimal (almost no settings) like the official Google Camera. If they work on your device, great. If something is not compatible, expect crashes or broken features.
If you have a Samsung device, specially one with a Exynos SoC, you may want to try miniuser123's versions. His versions are optimised for the Samsung Galaxy S7.
Another developer fixing problem on Exynos powered phones. Based on miniuser123, Arnova8G2, oganyan-arthur, etc.
SerJo87 tries to make Google Camera work better on the Xiaomi Redmi 3/Note 3 Pro.
oganyan-arthur, or just "arthur" to save space. His work is optimised for the Xiaomi Redmi 4X and based on the work of other developers (BSG, Arnova8G2, al_ro, etc).
alone_in_dark's versions are compatible with MIUI 9 and optimised for his Xiaomi Mi 5. Based on Arnova's work and probably compatible with other phones.
al_ro's work is valuable and ends up be used by other developers and also by Xiaomi users.
He does lot's of tests which results in a big number of apks posted on 4PDA. Since I don't understand Russian and because it's hard to keep up with so many updates, I don't post most of his apks here.
Saneklic makes apks from other developers compatible with the Xiaomi Redmi 5 Plus / Note 5.
APKs from fu24 are usually based on the work from other devs, but come with a toggle to switch between the main camera and wide angle/telephoto lens. Might be a good choice for phones with two rear cameras.
Ivanich is not longer active, but he did a very good job making some of the earlier BSG's versions compatible with the OnePlus 3/3T (and other devices). He added configurations, removed features that made the app crash, etc. His apks are based on Google Camera 4.4.x. Most versions are stable, but lack the features available on newer versions.
ArtZ does minimal changes to apks and tries to improve colour reproduction on his LG V6. No longer active.
Charles_l works on Camera NX, a port for Google devices. Check his thread for more info.
XDA / OnePlus 3
I'm a normal user, not a developer. I don't fix bugs or provide support. The download page is just a free mirror for files shared by BSG, Arnova8G2, and others.
I do this for a few reasons that can be found on the "about this site" page, but basically I was frustrated with my OP3's camera and decided to support this port.