I don't understand it at all, "converts fuel to syngas", but it doesn't accept petroleum, light oil, heavy oil, crude, natural gas, methane, nothing. And as far as I can tell the building doesn't have a 'recipe' and even factoriopedia in game has only 1 way to make syngas, which is "methane reforming".

This has me thoroughly stumped.

Put steam in the fluid slot and any fuel item. It will turn that fuel into syngas. If you want to make syngas from fluids directly you can do that by cracking down to methane and then using the methane reforming recipe.

Ah, steam, didnt even think to use that, maybe a change of the description of the gasifier to "Converts fuel and steam to syngas", not that I even know if anyone else even had trouble with it

Sure I can do that. Just for clarity: the idea of the gasifier is to turn fuel items into fuel fluids, allowing you to go backwards where the vanilla game only allows you to go from fluid fuels to solid fuel. Then once you have syngas you can reform into other oil products. Sure you can make syngas from methane but it's sort of counter-productive in a typical setup. But you may find other ways to use it, just letting you know my intent.