Justin, your screenshot shows that you're using a printer named EPSON FX 890 Ver 2.0 (and presumably, a printer driver that maps to this driver).
The Acrobat preview is smart. It can query the driver's (and therefore, the printer's) capabilities. And obviously, your printer does not support "borderless printing" (or "edge-to-edge" printing, or whatever a vendor may call it).
Some printer models do so. (Usually, these are more expensive. But they also get more common these days...) If they do, there is usually an option somewhere in the driver's user interface which lets you select borderless. Vendors usually recommend to not use it frequently, and therefore they don't enable it by default. It can make things messy, especially on inkjets or bubblejets.
As soon as you select such a setting in your printer, the gray preview borders in Acrobat's print dialog will disappear.
So there are 6 options for you:
- find the GUI location in your current driver which enables edge-to-edge printing (IF there is one),
- switch to a different printer which supports edge-to-edge printing (IF you have one) [*],
- buy a printer that supports edge-to-edge (IF this is a worthwile expenditure for you) [**],
- print your file somewhere (a friend?, a print bureau?) with access to an edge-to-edge printer,
- select for your printout to 'scale to printable area',
- live with the current defect.
[*] If you don't have a 2nd, 3rd, 4th... printer driver installed, you may do so just for the testing; you'll discover that every one of them will make Acrobat display its 'gray border of unprintability' with slighly different dimensions.
[**] Before you go shopping, you can also just install the respective printer driver for a new model first, and test if your Acrobat print preview shows a zero-width gray border.