Well, no. It's up to states how they comply with international obligations.
So, for example, the right to life implies that countries must have laws that prohibit murder. That doesn't mean that UK law that prohibits murder can never change. It's just that change must remain consistent with the Convention and its case law - otherwise it will be a breach of international law.
So if we decide, for example, that some of our distinctions between murder and manslaughter are outdated, remedying that is probably fine. If we decide that actually murder should be decriminalised if there is a full moon, or if the victim is named Sebastian - that law would be unlawful in international law terms.
So changing the GRA would be permitted, provided that the legal right to change gender (free from some of the constraints that the Court has since ruled to be violations of Article 8) remained.
The point is that scrapping the GRA entirely, with nothing to replace it, would be a breach of convention rights. And if the thing that replaced it was significantly weaker, that would also breach convention rights (since the GRA is already at the weaker end of what the ECHR requires).