That's all that is needed, editing HiredGangNames.txt.
The first line of the file indicates how many names are in the file (not the number of lines; it would be the number of lines - 1). Be sure you update it to the correct number and don't leave any blank lines, and you should be fine. Oh, also, be sure not to reduce the number of gang names below 20, assuming you haven't edited the "MaxRecruitList" value in config.xml (need to be sure it doesn't run out of gang names).
If you do come up with some decent new gang names, it would be nice to share them with everyone else.
EDIT: by the way, you might like the update gang name list we have included in 1.30.0:
36
Bloods
Kings
Counts
Vice
Lords
Crips
Cobras
Folks
Incorporated
Disciples
Young Guns
The Unforgiven
Wrecking Crew
Shadow Men
Wild Bunch
Red Hands
Black Sashes
Coyote Pack
Knockaround Guys
Waywatchers
Sewer Rats
The Lost
Sons of Saints
Wicked Messengers
Faceless Assassins
Skull Bangers
Widowmakers
Little Tykes
Hellhounds
The Warriors
Jolly Knifemen
Ironheads
Patrons of Purgatory
The Pink Ladies
Shining Monkeys
The Wanderers