It is not. If it is, you should explain that to every other expert and lawyer involved in litigating the differences, because they're operating under a delusion.
They are mutually inclusive licenses, because OSI licensed stuff can always be relicensed to GPL (or anything else, really), and because OSI says that GPL is also OSI (although I don't understand why.)
But they're different. You have to share your changes in GPL if you distribute them. That's it. That's the difference.
Not all free software is copyleft, and not all copyleft licenses are GPL compatible. But all free software is open source and vice versa. The four freedoms and the open source definition are intentionally equivalent.
There is all kind of somehow "open" software which might be OSI open source or even be GPL licensed but is de-facto not free software.
Like I have seen more then one case where GPL or similar was used to archive "open sourcing the software" while "making sure no on complying with law can benefit relevantly from it". E.g. publish a software under GPL which is strongly dependent all through the stack on some proprietary software which is GPL incompatible but has some pseudo decoupling though some interfaces. The company which wrote the software just uses a different license when they use it. Everyone else would have to spend impractical amounts of time to create adapters to make it actually usable under the GPL license. Sometimes not much less time then rewriting it from scratch. I have seen this pattern quite often. Through more often with AGPL then with GPL. Sometime it did feel that such theoretically but not practically free software likes (A)GPLv3 more the actually free software does like it (but I'm pretty sure this is a bubble).
Another adjacent term is “Source Available” for software that isn’t open source but you can get the source, like Unreal engine.
There is also Open Core for software that has an open source part and a proprietary extension. Like GitLab.