I tried a few apps on my last trip, all within Android Auto, and my absolute favorite so far was Here WeGo. It has free offline maps, real-time traffic jam warnings and reliable route guidance.
I was curious to see how Magic Earth would perform. Right at the beginning, the flow display for the next turn froze. I had to restart the app several times, but later I gave it another chance and entered an address in northern Germany. I would have been mercilessly misled. Fortunately, I know the exact location of the address and was able to switch in time. Also, with another address, its destination time was 1/2 hour more than Here WeGo (that is always realistic).
I also tried Organic Maps and OsMand, but I didn't like them. I'm glad to be back with Here WeGo and to be sticking with it. Visually, this app is also my first choice.
Regarding the initial question of whether GSF is needed: that wasn't relevant for me until now, as Android Auto needs it. So GSF was already installed. However, I suspect that GSF is not needed as a standalone.