My overflow drain has gas

[Delphinus]

Strange .... I don't think it should be this difficult, I wonder what I might be overlooking here.

Just a suggestion -- on my 20g tank, I have an overflow in the corner, same style as yours (i.e., two glass panes in the corner). It's just a small tank so I didn't bother with a durso or stockman or whatever, I just have a standpipe with a strainer (strainer not really needed, just to keep the {now defunct anyhow} wrasses from going down the overflow pipe since they liked jumping into the overflow). The water level differential is only about 2" at most (I'd have to measure to give you an exact figure, but basically, it's a small drop). Since there's no waterfall effect, it's pretty silent, silent enough for me anyhow.

Anyhow, whatever the outcome, there is a way to make it quiet, keep at it, you'll find the groove that works best with a little trial and error..