The Android documentation explains how to enable ProGuard in your builds (Ant, Eclipse, or Gradle). Once it is enabled:
ProGuard's shrinking step removes annotations if they appear to be unused in the code. If Gson uses the annotations, they probably won't be removed.
ProGuard's obfuscation step removes all annotations by default, unless the configuration specifies:
-keepattributes *Annotation*
The global configuration file in the Android SDK,
android-sdk/tools/proguard/proguard-android.txt
, contains this line, because most developers want to preserve annotations. If you comment it out, all annotations will be removed.