According to the site Beast Wars International The circled part in the above picture is a reference to Ranma. The gesture is "oyakusoku pozu" (promise pose) it is a sign of surprise or shock.
Megatron's hand gesture here is a reference to an anime show called Ranma 1/2' in which David Kaye featured. David plays the main girl's (Soun Tendo) father. Whenever confused in the show, they use that gesture.