Let me clarify my too brief answer. The food comps from both casinos can be used at either one. I have never attempted to charge a meal from the D to the Golden Gate or vice versa.
Reason to stay? The Golden Gate has tiny standard rooms with even smaller bathrooms. I hear the suites are very nice but have not stayed in one. The comped nights do no incur the resort fee. It is a small property with rooms I have found adequate to meet my needs (a comfortable bed with a functional bathroom). We are low maintenance people and my wife loves staying there also.
There are nicer properties downtown. I stay where I'm comped and will be staying at Fremont Casino in my upcoming trip. As stated previously I some how fell off thier radar in Jan/Feb but now have comped room offers for March/April, go figure.
I like the casino vibe at the GG/D. Golden gate only has one restaurant-dupars which is open 24/7. I never had a bad meal there although it is pricey for downtown.
Not sure if this is what you're looking for?