  The coefficient functions of the gluon condensate $<G^2>$, in the correlators
of heavy-quark vector, axial, scalar and pseudoscalar currents, are obtained
analytically, to two loops, for all values of $z=q^2/4m^2$. In the limiting
cases $z\to0$, $z\to1$, and $z\to-\infty$, comparisons are made with previous
partial results. Approximation methods, based on these limiting cases, are
critically assessed, with a view to three-loop work. High accuracy is achieved
using a few moments as input. A {\em single} moment, combined with only the
{\em leading} threshold and asymptotic behaviours, gives the two-loop
corrections to better than 1% in the next 10 moments. A two-loop fit to vector
data yields $<\frac{\alpha_{\rm s}}{\pi}G^2>\approx0.021$ GeV$^4$.
