HMMT November 2012 E9

Consider triangle $A B C$ where $B C=7, C A=8$, and $A B=9 . D$ and $E$ are the midpoints of $B C$ and $C A$, respectively, and $A D$ and $B E$ meet at $G$. The reflection of $G$ across $D$ is $G^{\prime}$, and $G^{\prime} E$ meets $C G$ at $P$. Find the length $P G$.