Electrical was never my strong suit...

I'm wiring up my boost gauge, this look OK? No issues?

No need to run the ground back to the fuse box, a good chassis ground is just fine.

Hard to give a real look over without knowing what the requirements of the gauge and sender are but the concept doesn't appear to have any issues I can see yet.

Why would a gauge need constant power? Some sort of memory?

What the directions said to do. I can post a pic of those too if you think it will help.

Should i ground the two separate or together?

Sorry, should add that the boost pressure sensor and gauge both have wires coming out of them separately

Can ground at the same point, pretty common to find factory stuff that way with a stack of ground wires on the same stud.

For a boost gauge it may not have much of an effect, but generally speaking it's a good practice to ground sensor and the thing that reads that sensor to the same point.

I'm also a little baffled that they want a constant +12V.

Unless, what they really mean is that they want one source of +12V for when the ignition is on and the "switched" 12V is to be tied in to the dash lights to either illuminate or dim the gauge readout with the rest of the dash. What's the brand/model?

It's an eBay brand from China called dragon gauge

Ground the sensor to the block.

Ground the gauge to the chassis under the dash.

Don't forget fuses.

Keep in mind that some companies - Subaru and Ford, there maybe others, use "ground switching". They switch the ground circuit and not the positive one.