Creating a custom GPT that pulls in the power of thousands of apps, such as Slack and Google Calendar, can be done using Zapier's AI actions and OpenAI's GPT-builder. To start, users need to set up their Zapier account and create the necessary AI actions within Zapier, which allows them to connect with various apps. They then build a custom GPT by entering instructions in the Create panel, fine-tuning it through the Configure panel, and adding conversation starters, files, and capabilities. The final step is to officially connect the GPT to its AI actions using a special URL, allowing users to share their custom chatbot with others. Once configured, users can test their GPT by asking questions about their schedule or other tasks, and refine it as needed. With this setup, users can build assistants that streamline their work across thousands of apps, boosting productivity.