Use log properties to expand: \gb(xy2)\g_b(xy^2)\gb(xy2).
\gbx+2\gby\g_b x + 2\g_b y\gbx+2\gby
\gbx+\gby2\g_b x + \g_b y^2\gbx+\gby2
2(\gbx+\gby)2(\g_b x + \g_b y)2(\gbx+\gby)
\gbx−2\gby\g_b x - 2\g_b y\gbx−2\gby