OpenAI encourages users to consider plugins as replacements and says it plans to help migrate GPTs to plugins. Other routes are covered in Custom GPT alternatives.
Source: OpenAI's Custom GPT retirement and migration FAQ. OpenAI says dates may change, so check the FAQ for your plan.
Keep reading
How to Move a Custom GPT to a Claude ProjectUse your exported instructions and files to recreate a GPT as a project-style workspace.How to Export Your Custom GPT InstructionsCopy your GPT instructions, conversation starters and settings so you have a clean backup.How to Back Up Custom GPT Knowledge FilesKeep originals of every knowledge file and decide how they will be loaded after migration.