You need to add these lines to your proguard so that gson class is kept while signing your app.android
##---------------Begin: proguard configuration for Gson ---------- app
# Gson uses generic type information stored in a class file when working with fields. Proguard google
# removes such information by default, so configure it to keep all of it. spa
-keepattributes Signature orm
# For using GSON @Expose annotation ci
-keepattributes *Annotation* rem
# Gson specific classes it
-keep class sun.misc.Unsafe { *; } io
#-keep class com.google.gson.stream.** { *; } form
# Application classes that will be serialized/deserialized over Gson
-keep class com.google.gson.examples.android.model.** { *; }
##---------------End: proguard configuration for Gson ----------