GApps (short for Google Apps) packages are add-ons for custom ROMs that are necessary in order to get Google apps such as Google Play Services and the Play Store on your device. Developers do not bundle Google apps with their custom ROMs because the apps are proprietary. Instead, ROM developers often recommend third-party packages that can be flashed separately on top of their builds.
As an example, the first builds of LineageOS 15.1 were recently released. At that time, Open GApps weren’t compatible with Android 8.1 Oreo (specifically, the SetupWizard APK was still for Android 8.0) and thus LineageOS recommended the MindTheGapps package maintained independently by XDA Recognized Developer javelinanddart as an add-on package.
Then, the first Android 8.1 Oreo-based Open GApps packages were released. Open GApps is by far the most popular choice to download Google Apps because of its frequent updates and flexibility. Users may have expected LineageOS to change their recommendation for Google apps packages from MindTheGapps to Open GApps, however, that hasn’t been the case. LineageOS still continues to officially recommend MindTheGapps, and only begrudgingly lists Open GApps as an alternative on their website.
So why does LineageOS still recommend MindTheGapps? The reason for that is because there can be issues with using a package that hasn’t been tested by the developer. For example, the LineageOS developers note that removing the stock dialer app (which happens when flashing the “super” or “stock” Open GApps packages) can result in emergency calls failing to work, as the framework relies on a very specific intent to send. Using a different dialer app such as Google Dialer, for example, can break emergency calling functionality.
Another example is that, according to a team member of LineageOS, the current Open GApps packages seemingly break Google Camera on several devices in LineageOS 15.1. Google Camera officially works on Nexus and Pixel devices, but it has been ported unofficially to many other devices.
Finally, another example here is that Open GApps does not support the upcoming backuptool v2 in LineageOS yet. backuptool v2 is a new version of an existing tool in LineageOS that backs up and restores /system add-on files. It now has better support for devices with A/B partitions.
To conclude, we’re not trying to rag on Open GApps here. It’s an excellent choice to get all of the latest Google apps and services, which is why it’s so popular among the community. The issues listed above aren’t really a fault with the packages themselves, but rather with how the ROM expects certain applications to be there but due to unexpected user modifications, they aren’t present. Custom ROMs are built and tested by their developers with their preferred packages, and the developers will not be able to guarantee stable functionality if users install different packages instead. Therefore, users should always flash the officially recommended GApps packages on top of any custom ROM builds.