You can ground the gauge on any part of the bare sheet metal frame that nearest to the gauge itself... you can use a self-tapping screw for the easiest option, but otherwise you can also ground it to any stud or bolt on the engine.
For power, there's no problem running straight to the battery, though the gauge will always be on unless it has an off button. I'd install an in-line fuse in accordance with the gauge manufacturer's recommendation if you were to go this route too. If you're installing the gauge in the cabin, then you can tap power for the gauge from either of the two slots marked IGN at the bottom left corner of the fuse panel once you swing it down. Those slots will accept a common blade-style crimp-on pin. By using an IGN slot, the gauge will only power up when the ignition is in RUN.
