I think you dropped a line on our support site but I thought I'd also reply here on a few of these:

Available on GOG
- Ashigaru Worker - Available in April each year, or at least when your PC thinks its April
- Dragon Worker - Available at Chinese New Year every year. Or at least when your PC thinks its Chinese New Year.
- Cynical Imp (Charity Skin & Kickstarter Themes) Available on GOG via purchase from Our site. You can only get a GOG code from those widgets so don't purchase on Steam :)

Not Available on GOG
- Prison Architect Worker - To my knowledge this relies on Steamworks to check you own Prison Architect on your steam account.

Not Readily Available
- Yogscast Worker - Only available for charity as part of the Yogscast Jingle Jam... I think it was also only Steam keys.
- Golden Worker - Only awarded for community contribution. I've given them away for maps, campaigns, art, cosplay, tournaments. Generally we don't run any contests any more so if you think you have something we'd like then for sure I'd be happy to give a code out... But now I think on it I don't know if we have codes for this on GOG :O

Not Available Anywhere

- Silver Worker - No one gets this other than Devs

I recognise there's a few completionists out there that would love to get their hands on the missing workers. I can ask around at some point to see if we can't make some of the ones that are hard to get or unavailable more available.

The three currently not available to you are a bit messy as they're all ultimately external redemptions which GOG doesn't do often (at the time their system wasn't set up for it). I think we only ended up doing the main charity one because it was available on Steam, and we got an agreement to get some GOG codes for it, and it was for charity.