If it's a shim gasket, in fact a metal gasket such as Cometic makes then it damn sure MUST be embossed. That is what makes the gasket seal.
When talking copper gaskets...that's a horse of a different feather. They are not embossed and you must use an o-ring to seal the combustion chambers. To do it correctly, you run the o-ring wire in the head and cut a receiver groove in the block.
Other than that, a shim gasket must be embossed. If not, it will not seal.
