Id like to be able to input or output gasses or heat via an Entangled Mekanism machine. I know it should be possible, I just don't know the steps to enable it in code.

What Minecraft version and modloader are you using?
For Fabric and for NeoForge I have to specify capabilities that the entangled block passes on explicitly, so likely that's just missing.

Using neoforge 21.1.80 and Minecraft 1.21.1 and Melanism 10.7.7.64. was All the mods 10, but I update mods manually and added a few extra

Making it possible to entangle a Mekanism nuclear waste barrel and chemical pipes would be very sweet

In Entangled 1.3.20a, it will now automatically register api providers for the entangled block for all capabilities. This should also make it work with Mekanism capabilties such as heat and chemical transfer.
Thank you for the suggestion!