One of my favourite elements of this hobby is being creative with the parts you have.
I received a package from eBay with parts for about 15 Cadian Imperial Guardsman. In the box was something extra. It had most of the parts required to make a Necron warrior, including the Gauss Flayer.

The above picture looks like an alien autopsy, which it kind of is. I don’t think I’ll assemble it, but I’ll use the parts. I’ll probably use the skull as part of a servo skull, replace a guardsman’s leg with the bionic foot, and give the Gauss Flayer to an Inquisitorial Acolyte. I don’t know if the Gauss Flayer will be tabletop legal, so if anyone objects I’ll just play it as a hot-shot lasgun that looks really cool.