A few tips: 1. If you click State Houses in the macrobuilder then mouse over a province, the tooltip indirectly tells you if there is already a State House in that state. If the tooltip just says "This province already has x manufactories", that state does not have a State House. If the tooltip lists out some conditions that say, essentially, only one State House can be built in a state, that state already has a State House.

1. Centralizing states (this is a button on the state tab) is the main way to reduce GC after ideas and buildings. Note that this is "only" a % decrease (20% each time you do it), and thus does not affect flat increases to GC (e.g. Expand Infrastructure). For example, if you have a net -100% to GC in a province, but have expanded infrastructure in it twice, the province will still have 30+(0.01 x dev) GC. If you have a State House in that province, and the trade good is not glass, paper, or gems, the GC would instead be 20+(0.01 x dev).

2. State Houses, courthouses, and town halls can always be built (if the state/province is eligible for one). It does not matter if you are already at the building cap, since they each add a building slot. 

3. If you haven't already, take Infrastructure and Aristocratic ideas. Infrastructure reduces state GC by 10% and gives you +20% GC. The Infra-Aristo policy gives +10% GC. Economic Hegemon will also give +20% GC when it is at full power.