So what you're looking for is a niche era/theme (the amount of gangster models is rather limited even in 28mm) and an even more niche scale. You're certainly not making it easy for yourself, and I really like that.
Did some googling for makers of 20mm in general (there really aren't a lot of them), and while the vast majority stick to soldiers and war, I found a few things, in addition to what you already found.
http://www.magistermilitum.com/scale/20mm.html#order=name&limit=36&p=1&dir=ASC&cat[]=59554&cat[]=61918 Magister Militum has a small range of civilians from the 1930-40s. While they're designed for British railways (thus all the conductors and Bobbies), the men in suits and the workers would look right at home in the lawless streets of Chicago or doing some shady work down by the concrete factory.
http://www.rafm.com/Merchant2/merchant.mvc?Screen=CTGY&Category_Code=COCM This range is old, and has a somewhat bigheaded style (which I like, but it's not for everyone), but contains a fairly large amount of gangster-era models, from mobsters and cops to dames and innocent bystanders. They even have a handful of vehicles and riders.
And as for vehicles and streets, have you tried searching for models in the 1:72 scale? Again, there's not tons of options when it comes to civilian stuff, but even a quick google search for "1: 72 car model kit" brought up quite a few suitable cars (like this beauty
https://www.modelhobbies.co.uk/shop/citroen-traction-avant-11cv-72273-p-17621.html)