When open source developers do that, they also include an explicit licensing information that lists cases when the usage is allowed and restricted. So even if the code is open source and licensed under GPL, its usage in a closed source product like ChatGPT is not allowed.
GPL code usage in closed source ChatGPT is allowed "for internal use"; it just would not be allowed to distribute binaries of ChatGPT that are closed source without making source available; also a GPL3 license violation to allow online access to a ChatGPT program that used GPL3 code without making source available.
