GSP provides tariff reduction for least developed countries but MFN is only for not discriminating among WTO members.
In 1971, the GATT followed the lead of UNCTAD and enacted two waivers to the MFN that permitted tariff preferences to be granted to developing country goods.
The United States, for instance, has excluded countries from GSP coverage for reasons such as being communist (Vietnam), being placed on the U.S. State Department's list of countries that support terrorism (Libya), and failing to respect U.S. intellectual property laws.
[1] Criticism has been leveled noting that most GSP programs are not completely generalized with respect to products, and this is by design.
That is, they don't cover products of greatest export interest to low-income developing countries lacking natural resources.