My overflow drain has gas

[Delphinus]

I've had similar issues when I had my overflow go straight down. Not sure if it's a complete solution but having a bend or something along the lines of a 45 so that the water can "slide" into the sump seems to lend itself better to a steady-state operation. There could also be maybe not enough air coming into the overflow. Anyhow just some random thoughts I hope might be helpful, good luck.