Want to use Gammu in your commercial product? As long as you obey license terms, you are very welcome to do so. The software is licensed under GNU General Public License version 2 and there is no option to relicense it for you (basically it is technically impossible to get agreement from all contributors to change the license).
However we can provide you some higher class support for your application. Should you need something like this, please do not hesitate to contact us either on some public place like mailing lists or Jabber or IRC chats or if you do not want to disclose you project, you can contact directly authors.
We do not provide any fixed price support packages, simply because every project is really different and it's best to arrange conditions separately to match everybody needs.
If you just want to appreciate level of support you got for free, you can also donate some money to the project.