Is Gwyneth Paltrow’s head shown?

The original ending which can be seen in the link provided by you, shows

a very quick flash of Gwyneth Paltrow's face before Mills shoots Doe.

It is quite possible that this is the source of confusion. But you can check this for your self (at 3:57 exactly) that this does look not like a severed head. Significance of the scene is that Mills thinks about his wife at that moment and her face flashes before his eyes, but this is Mills remembering his wife, not Mills imagining his wife's severed head. He thinks about her and it makes him so angry that he shoots John Doe. IMDB tells of DVD ending where

The DVD contains an alternate ending which features alternate takes of some scenes. It shows the delivery guy also hand Sommerset the truck registration. Afterwards, a wide shot of Mills is shown when John Doe reveals Mills' wife was pregnant, instead of the close up. There is no quick flash of Gwyneth Paltrow's face before Mills shoots Doe, and only one shot to the head is fired. There are no additional shots fired at Doe afterwards.

So there is no shot of the severed head in any version, only a quick flash of her face and that too is only in the theatrical version.

The ending is the central topic in this Entertainment Weekly article for the 20th anniversary of the movie:

The finale owes much of its edge-of-your-seat quality to the tension established earlier in the film. “Tension was built from the earliest scenes,” producer Arnold Kopelson writes in an email to EW. “When the audience is finally in the last scenes, it is the culmination of all that it has been carefully constructed it to be.”

Kopelson was one of the ending’s early opposers, but says he eventually came around. “We attempted many different endings and none worked,” Kopelson explains. “It needed this horrendous event to kick off the last sin, wrath.” He says he gave his okay when Fincher (Gone Girl, Fight Club) assured him that the audience would not see Tracy’s head in the box, leaving much of the horror up to the imagination. Kopelson credits Fincher with maintaining intensity at the film’s end.

Fincher also embraced creative risks. He employed shaky camera to enhance the action and drama and included a brief shot of Tracy just before Mills shoots Doe, decisions editor Richard Francis-Bruce applauds.

[...]

“People are convinced they see the head in the box,” Francis-Bruce says, “just because of Morgan’s performance.”

Up until last night, I've only ever watched Seven on cable TV, but last night I finally saw the whole thing from start to finish on Amazon.

Near the end of the box scene there is extremely brief (not too far above the subliminal level) of a close up of Gwyneth Paltrow's head in a blue filtered lens. I didn't expect it all and it was quite shocked since I've never seen it on the television edits.

My first impression was that it MUST have been her severed head since that is the subject of the surprise ending, the blue tone, and just flash it for a split second. The Next day, I replayed the ending and was able to pause it and discovered that it was was just a close up of her face while she was alive.

I'm sure the director didn't do this by accident. It isn't just Mills remembering his wife. We see the subject of what is in the box (a close camera shot of her head from life) rather than literally seeing a bloody severed head.

I found the whole sequence to be quite jarring, and I can understand why people argue over it. It is very hard to tell what is displayed on screen unless you slow the film down or pause it at the right moment.