Use the gsutil compose command:
gsutil compose gs://[BUCKET_NAME]/[SOURCE_OBJECT_1] gs://[BUCKET_NAME]/[SOURCE_OBJECT_2] gs://[BUCKET_NAME]/[COMPOSITE_OBJECT_NAME]
[BUCKET_NAME] is the name of the bucket that contains the source objects.
[SOURCE_OBJECT_1] and [SOURCE_OBJECT_2] are the names of the source objects to use in the object composition.
[COMPOSITE_OBJECT_NAME] is the name you are giving to the result of the object composition.