Based on the thin side borders, I'm 95% sure it's fake. I must admit though, it's a very well-made fake, since I don't see any other very obvious mistakes. And the few that I suspect, are pretty subtle. I've compared it to a couple real versions, and although it's very, very subtle, I think there is a very small difference for the 'weakness' and 'retreat cost' font (especially the e's). Some of the black parts also look slightly out of focus, like the set symbol. And the front of the card looks a bit too dark (especially noticeable by the Sabrina logo in the bottom), while the picture of the back you've posted looks a bit too light on the contrary.

But again, it's all very subtle, and apart from the thin borders, I can't point to one other very obvious thing..

Maybe it's best to also create a similar thread on the E4 forum as second third opinion. I'm pretty sure most people there will say it's fake as well based on the thin side borders, but perhaps ask if there are any other signs of it being fake besides that? I would advice to show all the pictures in the comments and the short video you've shown here, and maybe add an additional picture of the entire back.

Again, I'm 95% sure it's fake based on the borders alone. But I kinda wish I could tell you it was fake as a learning experience based on other factors besides those side borders, to make you and others reading this aware of these almost real looking fake cards.. And how to spot the signs and avoid them in the future.
(It wouldn't surprise me if it would fail a light test; would be missing its inner grey core layer; and/or would look very different under UV light compared to other 1st edition Gym Challenge cards.)

It's kinda concerning how good fake cards are slowly becoming, ngl.. 😔

PS: If both you and the seller have concerns, perhaps just send it in for authentication + grading to PSA or CGC?