Different caller ID for different area code

Is there a way to tell the gateway which SIP accounts to use for which appointments, we find that if you make the caller ID show a number specific to the area code it is much more likely to get an answer versus going to voicemail.

Not with one gateway. But you can achieve that by using multiple gateways.

You can specify the accepted area code in the gateway options (gateway > setup > options > Control tab > Scheduler preference). You can specify which area code of calls the gateway should accept or exclude. Voicent programs (BroadcastByPhone, AgentDialer, or AutoReminder), once connected to these gateways, will automatically schedule calls according to the area code preference.

Can you run multiple gateways on a single PC? And how does the licensing work with that? I currently have 1 license with 3 lines could I run 3 gateways with 1 line on each?

No, you cannot run multiple gateways on a single PC. For licensing, you have to have 3 separate software licenses for 3 PCs, each with Enterprise Edition.