I don't see a need for GPG at all. There are tons of options for sending email that is encrypted enough to lock out the Iranian government. Any popular mail service would work, and using several email addresses at different mail services would work best.

Also, organising a protest isn't about keeping individual messages secret. You can never keep something secret that you want thousands or hundereds of thousands of people to know about. It's all about getting the message across fast without the government blocking it.