darkSpyro

Is there a way to grind for Stardust and Gearbits?

Money cheat.

Don't know anything about a quick way to earn gearbits though.

Find all the hats....after you do that, that's just about all you'll get out of the wishing well.

Completing Live Wire lock puzzles will always give you stardust. Find a level that has one at the start, then just restart it as often as you like to get at least half a bar every time. Make sure to collect all 3 bolts and turn on any Portal Master rank powers that give you a boost for completing lock puzzles.

There is one at the start of level 26, Fungi Funhouse in cave on the right. And in the surrounding area are plenty of pinatas/presents/treasure chests to find to have a chance for even more stardust (or gearbits, gold, XP).

For Gearbits, for Sky Vehicles at least, I go to The Cloudbreather's Crag (Chapter 2), and straight to the Sky area. I can usually fully upgrade one of them in about four or five runs. For other vehicels, I haven't found a good spot. Although I do remember using the Chicken HQ Sea area for Gearbits when upgrading my Sea Shadow. Land...can't help you there. They just require sooooo many Gearbits. At least compared to the other types.

A good place to farm for land Gearbits is the tutorial level at the very start of the game before the first stage. I don't know if you can still play it when you start a second save file, or if it's only available at the start of a completely new file with no saves (I use the iPad version for this and just never complete/save at the first stage, then upgrade them on my PS4 game which has all the discounts unlocked).

You can get around 1000 land gearbits in about a minute (make sure you're drifting into the gearbits to make them worth 3 instead of 1), then quit and restart the tutorial. The intro can be skipped which saves loads of time. I got all my current land vehicles to max stats a few days ago this way.