Stock heatsink is aluminum. It's actually a repurposed PC northbridge heatsink. If you look around some people have found bigger versions. There is also the Argon THRML cooler if you want to go supercharged.
The stock heatsink and fan is capable of cooling the bitaxeGamma at stock hash frequency and voltage settings. If you aren't stable at the defaults, you should speak with the seller.
Because of the increased power of the BM1370 chip in the Gamma, there isn't much headroom for overclocking with the stock heatsink.
I will be getting the parts to mod the heatsink setup. Sidehack thought it could be the heatsink. As I look at your design I see a small chip maybe 7mm square. A 40mm heatsink rests on that.
So you have a balance act because the heat sink sits on top of the small chip and only has a set of 2 fasteners.
Since the heat sink is raised off the pcb having 4 points vs 2 is better.
If it works cool if not then it has an issue I don't understand.