We present L' band (3.8 $μ m$) MMT/Clio high-contrast imaging data for the nearby star GJ 758, which was recently reported by Thalmann et al. (2009) to have one -- possibly two-- faint comoving companions (GJ 758B and ``C", respectively). GJ 758B is detected in two distinct datasets. Additionally, we report a \textit{possible} detection of the object identified by Thalmann et al as ``GJ 758C" in our more sensitive dataset, though it is likely a residual speckle. However, if it is the same object as that reported by Thalmann et al. it cannot be a companion in a bound orbit. GJ 758B has a H-L' color redder than nearly all known L--T8 dwarfs. Based on comparisons with the COND evolutionary models, GJ 758B has T$_{e}$ $∼$ 560 K$^{^{+150 K}_{-90K}}$ and a mass ranging from $∼$ 10--20 M$_{J}$ if it is $∼$ 1 Gyr old to $∼$ 25--40 M$_{J}$ if it is 8.7 Gyr old. GJ 758B is likely in a highly eccentric orbit, e $∼$ 0.73$^{^{+0.12}_{-0.21}}$, with a semimajor axis of $∼$ 44 AU$^{^{+32 AU}_{-14 AU}}$. Though GJ 758B is sometimes discussed within the context of exoplanet direct imaging, its mass is likely greater than the deuterium-burning limit and its formation may resemble that of binary stars rather than that of jovian-mass planets. ; Comment: 14 pages, 3 figures. Accepted for publication in The Astrophysical Journal Letters