  We establish some of the basic model theoretic facts about the Gurarij
operator system $\mathbb{GS}$ recently constructed by the second-named author.
In particular, we show: (1) $\mathbb{GS}$ is the unique separable 1-exact
existentially closed operator system; (2) $\mathbb{GS}$ is the unique separable
nuclear model of its theory; (3) every embedding of $\mathbb{GS}$ into its
ultrapower is elementary; (4) $\mathbb{GS}$ is the prime model of its theory;
and (5) $\mathbb{GS}$ does not have quantifier-elimination, whence the theory
of operator systems does not have a model companion. We also show that, for any
$q\in \mathbb{N}$, the theories of $M_q$-spaces and $M_q$-systems do have a
model companion, namely the Fra\"{i}ss\'{e} limit of the class of
finite-dimensional $M_q$-spaces and $M_q$-systems respectively; moreover we
show that the model companion is separably categorical. We conclude the paper
by showing that no C$^*$ algebra can be existentially closed as an operator
system.
