What is international law?
International Law is the system of rules, norms and standards by which the world’s nations agree to be governed. Through a series of treaties and conventions, states have created a framework that governs human rights, diplomacy, war, trade, and advances the goal of global peace and security.
Unlike state-based legal systems, states must consent to be bound by international law. The rules established by human rights treaties, however, often become international norms. When this happens, other states can choose to pursue coercive action, including diplomatic pressure and economic sanctions, to encourage compliance. 
What is the Convention on the Rights of Migrant Workers?
The International Convention on the Protection of the Rights of All Migrant Workers and Members of Their Families aims to foster respect for the human rights of migrants. The Convention covers issues such as the right to adequate remuneration (art 25), freedom from torture and degrading punishment (art 10) and freedom of religion (art 12). 
The Convention has not been signed by China so it is not applicable in Hong Kong. Many migrant-sending states, such as The Philippines and Indonesia, have signed the Convention. Without the consent of migrant-receiving states, however, migrant workers are only offered limited protections through the Convention.
What is the GCM?
In 2016, the United Nations General Assembly voted to establish a compact for safe, orderly and regular migration.
The compact has 23 non-legally binding objectives to better manage all types of migration. It is motivated by values of ‘state sovereignty, responsibility-sharing, non-discrimination, and human rights.’ Although it is not legally binding, states such as New Zealand have recognised that it will guide local courts on their interpretation of their immigration legislation. 
What international law DOES apply in Hong Kong?
Although China has not signed or ratified conventions specifically designed to protect migrant workers, a lot of the provisions are becoming internationally recognised as human rights norms so they will still have an influence. Further, many of the provisions overlap with other treaties that China has signed. Hong Kong is bound by the ICESCR, which guarantees the right to rest and leisure (art 7(d)) and the ICCPR, which protects everybody from torture and degrading punishment (art 7).
Anybody with a grievance that has not been resolved through domestic means should thus be able to find a method of recourse through other such treaty bodies.
- Liam Cross
1. United Nations, International Law and Justice <https://www.un.org/en/sections/issues-depth/international-law-and-justice/index.html>. accessed 5th November 2019
2. William Slomanson, Fundamental Perspectives on International Law (Wadsworth, 2011) 4.
3. International Convention on the Protection of the Rights of All Migrant Workers and Members of Their Families, opened for signature 18 December 1990, 2220 UNTS 3 (entered into force on 1 July 2003).
4. United Nations, Global Compact for Migration <https://refugeesmigrants.un.org/migration-compact>. accessed 5th November 2019