You need to add these lines to your proguard so that gson class is kept while signing your app.

##---------------Begin: proguard configuration for Gson  ----------  

# Gson uses generic type information stored in a class file when working with fields. Proguard  

# removes such information by default, so configure it to keep all of it.  

-keepattributes Signature  

# For using GSON @Expose annotation  

-keepattributes *Annotation*  

# Gson specific classes  

-keep class sun.misc.Unsafe { *; }  

#-keep class com.google.gson.stream.** { *; }  

# Application classes that will be serialized/deserialized over Gson  

-keep class com.google.gson.examples.android.model.** { *; }  

##---------------End: proguard configuration for Gson  ----------