The RuntimeTypeAdapterFactory.java file originates from the gson-extra repository: https://github.com/google/gson/tree/main/extras
Link to the file: https://github.com/google/gson/blob/main/extras/src/main/java/com/google/gson/typeadapters/RuntimeTypeAdapterFactory.java

gson-extra is not distributed as it is "not versioned nor have API stability guarantees". Therefore it is copied as 
source code into this project. This is by design, as documented on the gson repository:
 - https://github.com/google/gson/issues/845
 - https://github.com/google/gson/issues/1104
 - https://github.com/google/gson/issues/1123
