Gold fringe is a decorative option. Although the U.S. flag code makes no reference to gold fringe, it has been used since 1835. In 1895 the fringe was added to the flag of all regiments of the U.S. Army.
There have been conspiracies about what significance this fringe might have, but it is nothing more than an optional decoration, or as described by the American Legion, an "honorable enrichment."
To only focus on the gold fringe trimmed flags which were displayed in the background of President Donald Trump's final address on January 19, 2021, as if that is a secret message about martial law, is to ignore that there were gold trimmed flags throughout his presidency.