Indeed, the Achievement buffer will contain all the data you want (if the statusCode returned is STATUS_OK). You can load the achievements "on-demand" from your activity (which extends BaseGameActivity per example) via:
getGamesClient().loadAchievements(new OnAchievementsLoadedListener() {
@Override
public void onAchievementsLoaded(int statusCode, AchievementBuffer buffer) {
if (statusCode == GamesClient.STATUS_OK) {
int achievementsCount = buffer.getCount();
[...]
}
buffer.close();
}
});
And do not forget to close the buffer.