2560x1440 resolution no longer available on external display

I had a similar problem once using an old VGA display, with a VGA to DVI adapter. The display's native resolution was 1440x900, but the options showing up in the display preferences seemed to include almost everything but 1440x900.

In my case, it turned out to be a defective VGA cable causing the problem. This seems less likely in your case, if you've been having the same issue with multiple different cables. Is there any common cable or adapter you've been using in all your attempts?

Your Viotek GN24CB is rated at 1920 x 1080 by the manufacturer and has VGA and HDMI ports. There is no information in the user manual suggesting higher resolution support, so your monitor is the limitation, not the 2016 MacBook Pro.

The Apple USB-C Digital AV Multiport adapter supports HDMI up to 1080p (1920 x 1080) at 60Hz (or 3840 x 2160) @ 30Hz with compatible displays). The Belkin USB-C to HDMI adapter can support up to UltraHD 4K (4096 x 2160) @ 60Hz for supported monitors.

I have an AcerK272HUL 27 inch monitor here that is rated as a WQHD (2560x1440) display. I couldn't achieve that resolution over HDMI, but did with the DisplayPort connection.