Matt,
I came up highway 145 from the south. As you come around the curve at the Ophir loop there is a dirt road that turns off to the left (forest road 625). You may have been on it if you went to Ames to hike into the butterfly trestle. If you take road 625 4 or 5 miles you will eventually run into road 623 that turns off to the west. It is near some buildings and it crosses the river so it shouldn't be too hard to spot.
Once you head across the river and start up the side of Sunshine you will be on the grade. Continue driving along this long tangent until you hit a hairpin curve that will take you up the mountain. The trail starts right at this hairpin curve. If you walk in a little ways there will be a trail marker with something about the Galloping Goose Trail.
If you are coming from the north you can also get to 625 just past Lime on highway 145. You will drive past Vance Junction and 623 is probably a couple miles down the road.