What does "G2" mean when used with X509 certficates and certificate authorities? For example "Google Internet Authority G2"?. I thought it was another way of specifying Class 2 (for organizations, for which proof of identity is required) but then see certificates such as "VeriSign Class 3 Public Primary CA - G2".

It couldn't be something so simple such as Generation 2 could it? Hope someone who woks for a CA can explain these naming conventions. Thanks in advance.
Yes, G stands for "Generation". When CA needs to get a new chain they just increment the generation number. 

For example GoDaddy's signatures:


G3 - sha256WithRSAEncryption
G4 - ecdsa-with-SHA384