If the code is in GMS it's not necessarily open source. The more apps rely on GMS, the less they can run on ungoogled devices. micro-g attempts to emulate what

If the code is in GMS it's not necessarily open source. The more apps rely on GMS, the less they can run on ungoogled devices. micro-g attempts to emulate whatever is necessary. If Google ports changes both to that and AOSP it's not a problem