Can I use a VGA monitor with the GeForce GTX 1080/1070/1060 since I only see a DVI-D port on the card?

You may use a DisplayPort to VGA adapter to connect your GeForce GTX 1080/1070 to an analog VGA monitor. Review the documentation of the DisplayPort to VGA adapter to make sure you are choosing the right adapter to support the resolutions and refresh rate of your monitor.