WHAT IT’S ABOUT Gloria Vanderbilt — mother of CNN’s Anderson Cooper — is one of the most famous women of the 20th century, first gaining fame during a 1930s custody battle that ultimately left her motherless, and in the care of her aunt, Gertrude Vanderbilt Whitney, founder of the Whitney Museum of American Art. She led a tumultuous and colorful life, much of it recounted in this Liz Garbus film, which also features Cooper who debriefs her throughout. This also recounts the tragedies, including the suicide of her son — Anderson’s brother Carter, in 1988.

MY SAY “Nothing Left Unsaid” will resonate with a viewer of a certain age — not the type of viewer who typically heads to HBO for their “Game of Thrones” fix. They’re older, or old enough, to have witnessed most of the 20th century and the role Gloria Vanderbilt played around its edge, as the most famous child of the century, then one of the most famous socialites. Like the Halley’s comet of celebrities, she soared back into view during the ’70s with her designer jeans empire after a relative period of celebrity quiescence. But these viewers will best remember the many decades that came before, especially the 1930s — which yielded “the poor little rich girl” and “most famous trial of the century.”

Garbus’ film is beautifully produced, but “Nothing Left Unsaid” can also be a little too languid, a little too languorous, even (if possible) a little too intimate. There’s an end scene, with Vanderbilt and Cooper standing over the graves of Wyatt Cooper, her fourth husband, and Carter, her son, after Anderson has kicked away at the ice covering the stone inscription. Then both mother and son stand silently for whole minutes, with “Stabat Mater” playing desolately in the background. It’s a painful scene, and one you almost feel you haven’t earned the right to witness. But the entire film bears witness to the fact that Vanderbilt helped create the entire age of celebrity: Why shouldn’t millions of strangers feel entitled to this one as well?

This really is in part a meditation on fame and how the ground under it has shifted over the decades, now nearly over a century. Vanderbilt never had her own reality series, but didn’t need one either. A battalion of Hearst newspapers and endless coverage in endless magazines accomplished the same trick. “Nothing Left Unsaid” is a sumptuous tableau of changing styles, fashions, trends, and manners.

You almost expect that great transition device, the spinning headline, to announce that Vanderbilt had become a ward of the court on Nov. 21, 1934. A striking beauty — and by the way, at 92, she still is — Vanderbilt was photographed endlessly, lavishly. Shot after shot, her dark hair and luminous eyes fill the screen, as the poor little rich girl turns into the enormously glamorous and eligible adult. She is married, then divorced — a terrible, abusive marriage to show-biz agent Pat DiCicco.

A marriage to conductor Leopold Stokowski followed. He yields the single best line of the whole film: “I knew I wasn’t going to stay married to Leopold,” said Gloria. “That’s when I met Sinatra.”

Through matrimony, Vanderbilt drifted in and out of the white-hot spotlight. A marriage to director Sidney Lumet ended in divorce, followed by her marriage to Cooper. She explained, “I’ve always had a fear of abandonment, but I’ll abandon you before you abandon me.”

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Meanwhile, Anderson Cooper, acting as the guide to her life, also drifts in and out of this film — which can’t quite decide how much attention to accord him, or how much he is willing to reveal. By the end, he reveals about as much as he ever has in public. The view is poignant, and moving.

BOTTOM LINE Fascinating, disjointed, moving, tiresome, elegant, tacky, fast, slow. There’s a little something for everyone here.