Example Android project showing how to properly configure ProGuard. ProGuard is a tool for 'shrinking' and obfuscating compiled classes. It can rename methods and fields, or remove them if they appear to be unused. This can cause issues for Gson which uses Java reflection to access the fields of a class. It is necessary to configure ProGuard to make sure that Gson works correctly.
Also have a look at the ProGuard manual and the ProGuard Gson examples for more details on how ProGuard can be configured.
The R8 code shrinker uses the same rule format as ProGuard, but there are differences between these two tools. Have a look at R8's Compatibility FAQ, and especially at the Gson section.
Note that the latest Gson versions (> 2.10.1) apply some of the rules shown in proguard.cfg automatically by default,
see the file gson/META-INF/proguard/gson.pro for
the Gson version you are using. In general if your classes are top-level classes or are static, have a no-args constructor and their fields are annotated with Gson's @SerializedName, you might not have to perform any additional ProGuard or R8 configuration.
An alternative to writing custom keep rules for your classes in the ProGuard configuration can be to use
Android's @Keep annotation.