If you cannot configure the firmware of the GSM modem or printer, you will probably need a small computer between them. If both can talk through serial ports, it can be very tiny - PIC, AVR (including a packaged version like Arduino), etc. If at least one needs USB, you might be better off with a headless board. This will range from $ 2 at the low end to perhaps $ 70 at the maximum. (There are also several Arm boards that run the embedded .NET platform, if that's your background ... how well they run it, I'm not sure)
If you need to do formatting or the printer depends on the computer to do most of the work, or your engineers are not familiar with the thinking of tiny embedded systems, you probably want something that can run the operating system - that is, a faster chip Arm with hundreds of megabytes of memory - think Beagleboard, plug-in computer, Chumby Hacker Board, etc. or one of those microservices that are mostly reprogrammed by x86 netbooks for better cooling. Depending on how careful you are, this puts you anywhere from $ 50 to $ 250.
You can also use an Android phone (select one with known USB host capabilities) and reset GSM capabilities, but you can spend a lot of time disconnecting Android components when all you really need is built-in Linux with full Libc. A more “linux-y” linux smartphone might be preferable if you find one that you expect will still be available.