GSM is not an air interface (CDMA is).
Most GSM networks, including the HSPA and HSPA+, use W-CDMA for the interface (yes - - a variant of CDMA).
So referring to a network as "GSM" is a bit misleading, as the air interface could be W-CDMA, TD-CDMA, or as is the case with Verizon, OFDM, which is a complete departure from CDMA, and more similar to WiMax.
I find it all quite confusing......it appears that in the future, the only network that is going to stick to CDMA is T-Mobile.
Even Sprint has indicated that they might be going the OFDM direction (perhaps alongside WiMax) for its future networks.
I don't think I'll ever really understand cell networks.....I just want my phone to download like broadband cable - - anywhere.....is that too much to ask?