What's MACE 12MM PLENUM SPACER HOLDEN COMMODORE VS VT VX VU VY ECOTEC L36 3.8L V6 made of and does it come with gaskets?

It's made up of heat-blocking, composite plastic and it comes with silicone sealant. To give you an idea in regards to the installation process, please refer to the link below:

https://www.maceengineering.com.au/WebRoot/ecshared01/Shops/mace/510F/07A7/ACFB/1D04/4CC3/AC10/000D/B2A1/PLENUM_SPACER_FITTING_INSTRUCTIONSmkii.pdf