The model posted a pretty big red flag Wednesday -- a video of her walking in NYC in a 🔥fit that screams revenge, with the telling caption ... "know your worth & onto the next."

If ya need an exhibit B, try the fact she no longer follows Glen on social media.

There was already some smoke about a split -- remember, Gigi had recently unfollowed Sydney as well, which many thought meant something was up between Sydney and Glen ... especially seeing how flirty they've been together on and off set of their new movie.

Now, we've been told Sydney is still very much with her fiancé Jonathan Davino -- and there appeared to be proof of that Tuesday when he took her dog for a walk.

Based on Gigi's social media activity ... it doesn't seem like she's buying that.

Her confirmation of the breakup comes a couple days after Glen and Sydney looked really cozy together at an event promoting the rom-com they shot down in Australia -- and it seems like Gigi didn't dig the public flirting, even if it was just to pump up the flick.

Does make ya wonder if Jonathan's, at the very least, thinking the same thing about Sydney and Glen's chemistry.

Gigi and Glen were together for 3 years.