Motherboard size has nothing to do with gpu size. Gpu's are only restricted in 3 ways. Physical limits. In other words, the size of the case and if hdd bays are removable. Power limits. The size of the psu and connectors. Technology limits. You won't be able to pair an ancient gpu with a modern cpu or vice versa or pair overpowered gpus with underpowered cpus.
Will the 380x fit the z97i, physically yes, tech yes. Is the psu correct? Unknown. Is the case physically able? Unknown.