This loop returns all gallery posts instead of just the ones with the ACF true/false field set to ‘true’. I could not find an example of this loop, so maybe I have the syntax wrong.
You’d want to use custom_field="featured_gallery" instead of acf_true_false="featured_gallery" since acf_true_false is not a valid query parameter that can be used on post loops.
If ever that still doesn’t work, you might want to test the actual output value of your featured_gallery field to see what it actually contains. There’s a chance you might need to use a different approach to filter your loop such as the one @Birkeholm suggested.