Gina came up with G-Train in High School. She thought it was funny, she mentioned it during an interview, and it stuck. Now the crowd does the "whoo whoo!" for her when she serves.
Mostly that stems from Rachel Schwartz, from back in the day when she would go back to serve and Steve would announce "Back to serve for Nebraska... Rachellll Schwartz!!" and the crowd would respond, "SCHWARTZ!!!"
Today we do it for many of the girls, but it originated with Schwarz. And Roof! Roof! Roof! probably predates everything. Then again the Ace/K cards predate that, so... it's hard to say where this all stems from. Back in the 80s when I was going to matches, we didn't do all this. But it's fun.