Let me answer your questions one by one:
1) Is there a way to modify an object in my gallery?
Unfortunately not. You can modify the object on Workspace and then put it back to Gallery.
2) Is it best to add the table space to the gallery object that contains the entity?
One gallery item may contain multiple objects, but you cannot edit those objects or add another object to the Gallery item.
If I understand correctly, you would like to bundle a tablespace and several entities that have the tablespace assigned together. This is actually possible, try this:
I have created a Tablespace and five Entities. All of them have the Tablespace assigned.
Select all the Entities and the Tablespace in Model Explorer.
Right-click and select Add Object to Gallery | New Gallery or any already created gallery**.**
All of the objects have been added to one Gallery item.
Open the target model, right-click the Gallery item and select **Add to Active Model **(this is the only way to add multiple objects of different types to the model).
The Entities and the Tablespace have been added to the Model (they can be found in Model Explorer). All the Entities have the same Tablespace assigned.
3) Is it possible to add child objects to an existing gallery object
I know what you’re thinking, but unfortunately Gallery items do not work like that. You can only create Folders to organize your Gallery items.
4) If possible, I’d like to do this so that the objects in my model can be updated once the gallery object is updated.
As explained before, you cannot edit objects in Gallery, which means this is not possible.
I hope the described workaround will be enough for your purposes. If you have any more questions, go on and ask.