When you create a drawable without specifying a GC or any GC attributes, the drawable gets the default GC for that depth and screen. This default GC has all the default GC attributes, except that foreground is black and background is white. So in fact, you can often create a drawable and draw into it without worrying about GCs or graphics attributes at all. You are not permitted to modify the default GC.
When a drawable gets the default GC, it is actually sharing that GC with all the other drawables that use the default. And if you specify a GC when creating a drawable, or if you change the GC of a drawable, you are sharing that GC with any other drawables that use that GC. If you should modify a shared GC, all the drawables that use that GC will feel the effect. However, if you modify the graphics attributes of a drawable, only that drawable will feel the effect. This is accomplished by cloning the shared GC, modifying the new copy as specified, and installing the clone as the drawable's GC.
It helps to think of two different levels of use. If you never create a GC, and only modify graphics attributes of drawables, then you can think of the graphics attributes as belonging to drawables directly. The graphics attributes of a drawable will only change when you do it directly. However, if you create GCs, install them in drawables, and modify them directly, then a drawable's graphics attributes can change out from under it.
This can be very useful. You can change many drawables' graphics attributes all at once by changing their shared GC. And you can still change one of these drawables' graphics attributes directly, without changing the others. However, once you do this, that drawable will no longer share GCs with the other, so changes to the shared GC will no longer be felt by that drawable.