I will buy 2 GSM modems. The 1st unit is GSM modem Type SGDK from MOBITEK, and the 2nd GSM modem from another vendor. Then I can use the SMS Gateway Development Kit for both GSM modems?

No, because MOBITEK SMS Gateway Development Kit only works with GSM modem Type SGDK. It will not with GSM modem that non-Type SGDK.

Even though 2 modems having the same brand and they look the same, in actual fact they have different firmware. And some firmwares have problem working with our SMS Gateway Development Kit. We test all modems sold by us to certify they can work with the SMS Gateway Development Kit.