We present the first measurement of σ(gg→tt)/σ(pp→tt). We use 0.96  fb−1 of s√=1.96  TeV pp collision data recorded with the CDF II detector at Fermilab. Using charged particles with low transverse momentum in tt events, we find σ(gg→tt)/σ(pp→tt)=0.07±0.14(stat)±0.07(syst), corresponding to a 95% confidence level upper limit of 0.33, in agreement with the standard model next-to-leading-order prediction of 0.15±0.05.