Synthetic and natural gemstones are both real gem stones. They both have the same chemical composition. As you stated the synthetic variety of a gem stone is grown in a laboratory and the natural gem stone was created over time by geological processes. To call them real synthetic and real natural is redundant and unnecessary. When I am referring to a synthetic gem species, I usually use the term lab grown. Partly because most people equate synthetic with unreal.
Simulants on the other hand are used in jewelry because they have some of the properties of natural stones such as dispersion which gives them their fire. An example of a simulant is CZ which has a similar dispersion as diamonds but is much less expensive to produce and cut for jewelry. Another term to use for simulant is immitation.