Is One Piece Film Gold canon or not?
As you said, based on the time frame alone the movie cannot be placed anywhere in the manga story line since after Doflamingo was defeated the crew separated once more (the only possibility where this could happen is "far" into the future, after the upcoming reverie/Wano arc).

Also it's worth noting that Oda was only the executive producer on the film (he supervised the art direction), he did not write the story for it.

Generally everything outside the manga is considered non-canon unless Oda specifically declares something as belonging to the main story.