  We compute next-to-leading order virtual two-loop corrections to the process
$gg\to ZZ$ in the low- and high-energy limits, considering the contributions
with virtual top quarks. Analytic results for all 20 form factors are presented
including expansion terms up to $1/m_t^{12}$ and $m_t^{32}$. We use a Pad\'e
approximation procedure to extend the radius of convergence of the high-energy
expansion and apply this approach to the fini\ te virtual next-to-leading order
corrections.
