When the six girls entered in tonight's McDonald's Capital Gymnastics Invitational at George Washington University's Smith Center perform, they might have a little extra incentive since U.S. national junior champion Chris Riegel, 15, will be watching.

"When I mention his name to Jackie (Cassello) and Shari (Mann), they giggle uncontrollably," said Greg Weiss, the coach at MG Gymnastics in Rockville, and the force behind tonight's 7 p.m. competition. "All the little girls are in love with him."

Riegel, of Reading, Pa., will do a special exhibition of a tsukahara vault with a full twist. "It's a world-class movement," said his coach, Larry Moyer. "Its not the vault itself that is so great but the way he does it . . . so powerful and dynamic for a boy his age."

Riegel approaches the movement with little concern. "I've done it before," said the 5-foot-3, 116-pound high school junior. He will joined in the exhibition by Bart Conner, the country's No. 1, amateur male gymnast since Kurt Thomas called it quits, and Wally Miller, ranked eighth.

The six young female gymnasts doing movements such as the Hect back, Thomas flair and Comaneci dismount in elite competition will include Tracee Talavera, 13, of Danville, Calif., ranked No. 2, and Karen Kelsall, a member of the Canadian Olympic team.

Top-ranked Julianne McNamara has been sidelined by the flu.

Cassello, 14, the 1979 Pan American Games gold medalist, and Mann will represent Weiss' club.

Lisa Zeis of Buffalo, ranked eighth in the country, and Susie Van Slyke of Willingboro, N.J., ranked 16th, round out the list of competitors. s