As pointed out above, most roads are graded at <8% for any significant distance (>1 km). Short bursts require basically grinding it out. Mt. Diablo is the standard local torture test. The final wall is about 200m at 17% grade. With a 34/28 compact I can alternate between standing and spinning for the ~1 min climb duration, and I doubt I'm in significantly better shape that the OP.
I also mountain bike, and climbing on a road bike is significantly easier, because its totally non-technical. On a MTB, you frequently have local obstacles that require rolling over, where roads are basically graded smooth, so the average grade is a good representation. Think of it like climbing stairs vs walking up a ramp.
I suspect there's something wrong with your GPS. On a 30% grade, you're going to fall over backwards. A good number of pedestrians would literally be on hands and knees up a 30% grade. As a good estimate, what the total elevation gain and distance of the climb?