Is this gunk normal on a refurbished PC’s GPU?

Yep, it looks like someone tried to use thermal paste instead of thermal pads, which can cause a big mess and won't work properly. Since you bought this from ACER, I wouldn't waste time trying to fix it. Returning it is probably the best option.

There have been reports of Gigabyte GPUs leaking thermal gel, but judging by your description, it sounds like someone might have done a bad job reapplying it. If the system is still not functional, I'd recommend returning it. It's not worth further hassle when you could be dealing with a non-operational system.

It looks like thermal gel to me. If the GPU is a Gigabyte model, it’s usually not a major issue. You might want to check if it’s sitting properly in the slot and see if the motherboard shows any signs of issues, like the red light you mentioned. But honestly, it seems like this PC might have some underlying problems if it wouldn’t boot up initially.