SO, I verified my builder profile, and set the access of my GPT as accessible to everyone. Yet testing on anonymized browser or another browser still shows “Requires ChatGPT Plus” message. Tried to toggle back and forth, unpublish and republish GPT, change browsers, etc. - still the same error.
Use of GPT-4 and use of custom GPTs within ChatGPT requires one to be a subscriber to ChatGPT plus.
Your creation of a GPT link doesn’t let you give out free ChatGPT Plus subscriptions to the world. 
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If you want to make AI applications available to people by your own application or your own web site (at your expense for the actual language data) you can look more into coding for the OpenAI API if you are in to programming.