Gwen Berry Protests Nat'l Anthem ...

Gwen Berry -- a U.S. hammer thrower who'll compete at the Tokyo Olympics -- partook in what event honchos have said they won't tolerate come game day ... national anthem protests.

Berry, who's Black, did well enough Saturday at the 2021 U.S. Olympic Track & Field Team Trials in Eugene, OR to qualify for her second Olympics as a pro athlete ... but when it came to accepting her bronze medal on the podium -- "The Star-Spangled Banner" began playing.

She wasn't having any of it ... with several photos capturing the awkward moment, in which GB refused to place her hand over heart, and even turning away from the flag -- unlike her fellow competitors, who did the exact opposite.

At one point, while…