My GM doesn't have rules for calling in a planetary bombardment while the team is planet side. Does anyone know in which book they would be, or what they would be?

Check Battlefleet Koronus, page 133. The gist is that you can call down a strike with either macrobatteries or lances and simply hit a target. Anything within a relatively small blast radius is instantly killed (before burning Fate, of course), and things within a larger radius instead take a lot of damage.

The most important thing to note about a "good" planetary bombardment is that they very thoroughly devastate an area, but take quite a bit of time. In Ship combat terms, it can take 4-6 Strategic Rounds to finish bombarding even a "small" city. If you're not going for completely thorough devastation you can probably do quite a bit of damage, but your GM should consider giving things on the surface a chance to survive.

I suppose part of this will be a matter of scale, from a ship to a massive, land-based installation, with a mountain-sized reactor SOMEWHERE, but does it cover how planetary void shields interact with bombardments? I've always thought it was kind of weird how a single volley of most things will topple your void shield, at least temporarily, on your city-sized ship (suppose that's to cover the "only one shot a turn" ships), but these same shields are expected to protect my castle from orbital strikes. Are they just that much stronger, drawing that much more energy, or does the book not even remember that they are a thing? Only War has the Duke's castle I assume protected by multiple void shields, and numerous macro-emplacements, maybe even some lance cannons, but they claim that the Navy, with a small fleet, still can't just fly in, and level Severus' castle from space. I would think each ship could fire once, toppling his shields, and then fire again, slamming his fortress, while each ship would also have 1-3 void shields, and be able to weather a few hits from the land-based defenses, yet they don't, and I don't know if it's just the shoe-horning of the writers, sort of their continued way to make sure the story never ends, unless YOU make it happen, or if there is some difference between my city, and your city-in-space?

The city in space also has to devote a lot of power to keeping the engines going, keeping the macrobatteries firing, keeping the life support systems operational, keeping inertial dampeners from failing... there's a lot of strain on the main engine bay, and you can't mount overlapping shield generators, so that explains to me why a ship's shields would fall apart.

For a planet, unless you bring an absolutely massive fleet I've always assumed that a planetary generator would be able to protect the city. If you look at the Frozen Reaches, the capital city of Damaris is protected by a planetary shield that does keep the worst of an Orkish invasion away. However it doesn't go "through" the ground, so it stops just high enough that an invading army can pose a real threat. Which is, in my opinion, genius.

I imagine a sufficiently devastating attack would overload the shields, but it also justifies why everything comes down to a face-to-face battle.

On planets the generators for the shields are the size of ships or countless generators city wide (you can build a lot in a hive city). The height reason is the shield on cities focus all their force in a 180 degree area further strengthening it. It also alloys air in and out (no to mention cars). Is does tend for the defensive wall setup (in a way anchoring the shield) that leads to the land invasion.

I also read in one of the story books (not sure which one) but on planets the actual generator is too big for a ship to mount and while only large cities like forts and hives have one at all.

On my original question thanks for the book reference. My situation was I wanted to blast a building full of orcs rather then let them chases us (we were on the planet already) or at least blast near enough to distract them.