As Mark notes, the result is quite old, possibly dating back to the 90s. However, a quick search yields this paper, which briefly discusses this result. Then it cites this thesis for proofs.
Technical details aside, the intuition why this is true is quite simple. A complex vector $v$ can always be transformed into a larger real vector $[v]_r$, which contains the same information. The easiest way of doing this is to create a vector of double size, where half the entries contain the real part and the other half the imaginary part of $v$.
Similarly, we can figure out how to transform a complex gate's matrix $G$ into larger real matrix $[G]_r$, such that $[G]_r[v]_r = [Gv]_r$. This should always be possible because everything is a linear transformation.
If we do this for every gate in a universal gate set, then we can transform any quantum circuit into an equivalent one where we always remain within the real space. The only trouble I foresee, is that measurements are non-linear processes and getting their equivalence might require some thought.