get_query_var() only works with the Core WP_Query object:
Retrieve public query variable in the
WP_Queryclass of theglobalobject.
$wp_queryhttps://codex.wordpress.org/Function_Reference/get_query_var
Your mistake is a simple PHP one: The key is foo, not bar.
$foo = $_GET['foo'];
echo $foo;
But please do not echo user supplied data to the page without sanitizing it.
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