AdMob is an obvious choice for Google, as it's built mobile and in-app advertising (especially on the iPhone) into a $100 million a year business, enough to get Brin and Page personally involved in the courtship. Says TechCrunch:
Google is gunning hard to dominate mobile Web advertising and AdMob has an early foothold in the display side. [...] Google’s purchase price tells us it thinks the opportunity for mobile display ads is in the billions of dollars, at the very least.
Gizmo5, rather than advertising, could help round out Google's services portfolio. Again, TechCrunch has the details:
Google Talk allows voice calls between users but has no PSTN link to allow incoming or outbound calls to real phones. Gizmo5 does this well already. [...] And Google Voice is a great VoIP and phone identity service, but they have no endpoint for calls. Gizmo5, which by the way already integrates with Google Voice, is a soft phone end point for Google phone users.