In order to connect a DVI-D monitor to the DisplayPort port on USB-C Dock, an active DisplayPort to DVI-D adapter or cable is required. A regular passive DisplayPort to DVI cable will not work.
For example, the LG 27MP38VQ monitor has both VGA and DVI-D inputs. You can connect it to the USB-C Dock via a DisplayPort to VGA cable, or an active DisplayPort to Female DVI-D adapter to a passive DVI-D to DVI-D cable.
The following is the setup information: Apple MacBook 2016 — CalDigit USB-C Dock — Active DisplayPort to DVI adapter — DVI cable — LG 27MP38VQ Monitor