So I know common practice is to run CSST underground through PVC conduit, can you use ABS instead ? Or even 636 ?
Yes, the Gastite manual says,
Quote4.9 UNDERGROUND INSTALLATIONS
a) Gastite/FlashShield+ CSST shall not be buried directly in the ground or directly embedded in concrete (e.g. slab on grade construction, patio slabs, foundations and walkways). When it is necessary to bury or embed Gastite/FlashShield+ CSST, the tubing shall be routed inside a non-metallic, watertight conduit that has an inside diameter at least 1/2 inch larger than the O.D. of the tubing (Fig. 4-87). For ends of the conduit installed outdoors, the conduit shall be sealed at any exposed end to prevent water from entering.
Based on Admin's quote, there, I think the advantage to using electrical PVC conduit would be the much longer radius elbows. These would be much more forgiving on the tubing as well as easier to install.