The NT itself attests to numbers of Gentile "God-fearers" attending synagogue long before the Gospel went forth--it was these Gentile God-fearers, in fact, who paved the way for the success of the Gospel among the Gentiles (see Acts 13 in particular).
The issue in Acts 15 and Galatians was not that all of the twenty-four or so sects of Judaism were united in the belief that all Gentiles must be physcially circumcised in order to be "saved," but that so many were turning to the Gospel that it was messing up the accepted social boundary markers. The seemingly expedient solution was to go with the view that no Gentile had a place in the World-to-Come, but that they could have one if only they became Jewish by the rite of circumcision. Had the Gentile mission been less successful, I doubt the issue would have been raised.
Shalom.
I was just joking, but thanks for the history lesson. I never knew the ancient Jews looked for converts.