Change md5Sum to etag (#4399)

cmd/gateway-gcs.go

@@ -164,19 +164,13 @@ type gcsGateway struct {
 
 // newGCSGateway returns gcs gatewaylayer
 func newGCSGateway(args cli.Args) (GatewayLayer, error) {
-	if len(args) != 1 {
+	if !args.Present() {
 		return nil, fmt.Errorf("ProjectID expected")
 	}
 
-	profile := "default"
-	// TODO: don't know where to set profile yet
-	_ = profile
-
 	endpoint := "storage.googleapis.com"
 	secure := true
-
 	projectID := args.First()
-
 	ctx := context.Background()
 
 	// Creates a client.
@@ -485,8 +479,8 @@ func (l *gcsGateway) PutObject(bucket string, key string, size int64, data io.Re
 		teeReader = io.TeeReader(teeReader, sha256Writer)
 	}
 
-	md5sum := metadata["md5Sum"]
-	delete(metadata, "md5Sum")
+	md5sum := metadata["etag"]
+	delete(metadata, "etag")
 
 	object := l.client.Bucket(bucket).Object(key)