diff --git a/errors.go b/errors.go
index 5d856b1..286f838 100644
--- a/errors.go
+++ b/errors.go
@@ -9,6 +9,14 @@ package pushreceiver
 
 import "github.com/pkg/errors"
 
+type GCMError string
+
+func (err GCMError) Error() string {
+	return "GCM error: " + string(err)
+}
+
+const ErrGCMDeviceRegistration = GCMError("PHONE_REGISTRATION_ERROR")
+
 // ErrGcmAuthorization is authorization error of GCM.
 var ErrGcmAuthorization = errors.New("GCM authorization error")
 
diff --git a/events.go b/events.go
index e895759..c08a2d8 100644
--- a/events.go
+++ b/events.go
@@ -55,6 +55,10 @@ type MessageEvent struct {
 }
 
 func newMessageEvent(data *pb.DataMessageStanza) *MessageEvent {
+	appID := data.GetAppID()
+	if appID == "" {
+		appID = data.GetCategory()
+	}
 	return &MessageEvent{
 		PersistentID: data.GetPersistentId(),
 		From:         data.GetFrom(),
@@ -65,7 +69,7 @@ func newMessageEvent(data *pb.DataMessageStanza) *MessageEvent {
 		Token:        data.GetToken(),
 		RegID:        data.GetRegId(),
 		RawData:      data.GetRawData(),
-		AppID:        data.GetAppID(),
+		AppID:        appID,
 	}
 }
 
diff --git a/gcm.go b/gcm.go
index 6d83a02..b43de72 100644
--- a/gcm.go
+++ b/gcm.go
@@ -27,6 +27,22 @@ type GCMRegistrationOpts struct {
 	Expiry     time.Duration
 }
 
+func setGCMAppID(values url.Values, appID string) {
+	if len(appID) == 32 && strings.Trim(appID, "abcdefghijklmnop") == "" {
+		values.Set("app", appID)
+	} else {
+		values.Set("app", "org.chromium.linux")
+		values.Set("X-subtype", appID)
+	}
+}
+
+func NewGCMAppID(authorizationEntity string) string {
+	if _, err := strconv.ParseUint(authorizationEntity, 10, 64); err != nil {
+		return "wp:" + strings.ToUpper(uuid.New().String())
+	}
+	return uuid.NewString()
+}
+
 func RegisterGCM(ctx context.Context, authorizationEntity string, creds GCMCredentials, opts *GCMRegistrationOpts) (*FCMCredentials, error) {
 	values := url.Values{}
 
@@ -34,11 +50,11 @@ func RegisterGCM(ctx context.Context, authorizationEntity string, creds GCMCrede
 	if opts != nil && opts.AppID != "" {
 		appID = opts.AppID
 	} else {
-		appID = "wp:" + strings.ToUpper(uuid.New().String())
+		appID = NewGCMAppID(authorizationEntity)
 	}
 
 	if opts != nil && opts.InstanceID != "" {
-		values.Set("appId", opts.InstanceID)
+		values.Set("appid", opts.InstanceID)
 	}
 
 	if opts != nil && opts.Expiry != 0 {
@@ -46,10 +62,9 @@ func RegisterGCM(ctx context.Context, authorizationEntity string, creds GCMCrede
 		values.Set("ttl", ttl)
 	}
 
-	values.Set("app", "org.chromium.linux")
+	setGCMAppID(values, appID)
 	values.Set("scope", "GCM")
 	values.Set("X-scope", "GCM")
-	values.Set("X-subtype", appID)
 	values.Set("device", fmt.Sprint(creds.AndroidID))
 	values.Set("gmsv", strings.Split(chromeVersion, ".")[0])
 	values.Set("sender", authorizationEntity)
@@ -61,18 +76,14 @@ func RegisterGCM(ctx context.Context, authorizationEntity string, creds GCMCrede
 	if err != nil {
 		return nil, errors.Wrap(err, "request GCM register")
 	}
-	defer closeResponse(res)
-
-	data, err := io.ReadAll(res.Body)
+	subscription, err := parseGCMResponse(res)
 	if err != nil {
 		return nil, errors.Wrap(err, "read GCM register response")
 	}
-
-	subscription, err := url.ParseQuery(string(data))
-	if err != nil {
-		return nil, errors.Wrap(err, "parse GCM register URL")
-	}
 	token := subscription.Get("token")
+	if token == "" {
+		return nil, errors.New("GCM register response missing token")
+	}
 
 	return &FCMCredentials{
 		GCM:   creds,
@@ -83,10 +94,9 @@ func RegisterGCM(ctx context.Context, authorizationEntity string, creds GCMCrede
 
 func UnregisterGCM(ctx context.Context, authorizationEntity string, creds GCMCredentials, appID string) error {
 	values := url.Values{}
-	values.Set("app", "org.chromium.linux")
+	setGCMAppID(values, appID)
 	values.Set("scope", "GCM")
 	values.Set("X-scope", "GCM")
-	values.Set("X-subtype", appID)
 	values.Set("device", fmt.Sprint(creds.AndroidID))
 	values.Set("gmsv", strings.Split(chromeVersion, ".")[0])
 	values.Set("sender", authorizationEntity)
@@ -99,9 +109,32 @@ func UnregisterGCM(ctx context.Context, authorizationEntity string, creds GCMCre
 		return errors.Wrap(err, "failed to unregister with GCM")
 	}
 
-	if res.StatusCode != http.StatusOK {
-		return errors.New("failed to unregister with GCM")
+	response, err := parseGCMResponse(res)
+	if err != nil {
+		return errors.Wrap(err, "read GCM unregister response")
+	}
+	if response.Get("token") == "" && response.Get("deleted") != appID {
+		return errors.New("GCM unregister response missing confirmation")
 	}
 
 	return nil
 }
+
+func parseGCMResponse(res *http.Response) (url.Values, error) {
+	defer closeResponse(res)
+	if res.StatusCode != http.StatusOK {
+		return nil, fmt.Errorf("unexpected HTTP status %d", res.StatusCode)
+	}
+	data, err := io.ReadAll(res.Body)
+	if err != nil {
+		return nil, err
+	}
+	values, err := url.ParseQuery(string(data))
+	if err != nil {
+		return nil, err
+	}
+	if message := values.Get("Error"); message != "" {
+		return nil, GCMError(message)
+	}
+	return values, nil
+}