We use butane and "type L" copper, which is thicker than normal copper
for water.

I am told that type L copper is ok for gas as long as the pipe is safe
from being hit, and with the advantage that copper is more resistent to
corrosion. If the pipe is exposed to physical damage, then galvanized
should be used.

I called my local AHJ, and they pointed me to an official norm that
states to use twice the normal-use pressure for 30 min.

They also pointed me to a certified expert, which I am actually going
to hire to do the testing.

Thanks every body that responded. Your advice was very helpful!

Cheers