In your last picture it appears to me that the pipe leaving the M and going to the gutter is at an uphill angle. It needs to be running down hill all the way.

Have you increased the water flow in the GB to the point of almost overflowing your GB? I still think you need more flow into, maybe a longer center stand pipe & a downward run from your M all the way to your gutter.

One of the way to achieve is to come off the center tee on your M with a 45<, a longer run to the end of your GB, all at a 45 degree angle. Your siphons "fire" by the use of gravity...a longer run would increase your chances.

Before you scrape everything I would also try running the stand pipe on the M straight down & out the bottom of your GB. Than a 90< to your gutter, make sure that your tail pipe still runs downhill and a 90< on the end of the discharge pipe. Keep a 90< on the gutter end, turned at a slight upward angle.

Don't forget about increasing your water flow into your GB's...as long as it works without overflowing it !