Jamieson, Fausset and Brown on 3 John 1:7

7. his name's sake--Christ's.

went forth--as missionaries.

taking nothing--refusing to receive aught by way of pay, or maintenance, though justly entitled to it, as Paul at Corinth and at Thessalonica.

Gentiles--the Christians just gathered out by their labors from among the heathen. As Gaius himself was a Gentile convert, "the Gentiles" here must mean the converts just made from the heathen, the Gentiles to whom they had gone forth. It would have been inexpedient to have taken aught (the Greek "meden" implies, not that they got nothing, though they had desired it, but that it was of their own choice they took nothing) from the infant churches among the heathen: the case was different in receiving hospitality from Gaius.