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SAG-AFTRA Prez and '90210' alum Gabrielle Carteris[1] knows Donald Trump[2] resigned to dodge punishment from the union, but says he could still get a rude awakening.

Gabrielle joined "TMZ Live" Friday to address the former President's letter to her, which as we reported contained the typical Trump beats -- petty insults, boasting and of course ... a claim he's been treated unfairly.

GC points out the timing of his resignation[3] was just a day before he could have been disciplined and possibly expelled[4] from the union -- so, she believes he simply didn't want to go through that.

Well, turns out Trump's not really in the clear just because he quit[5]. Gabrielle laid out what SAG-AFTRA could still do to prevent him from ever rejoining the union.

As for why she felt action had to be taken -- Gabrielle gave a very impassioned explanation of what she saw play out[6] on Jan. 6, and how it threatened lives ... including those of union members.

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