Serial Over IP or USB over IP?
I don't know what "Serial over IP" is -- TCP/IP provides a bidirectional socket connection that can carry many different application protocols. I've never seen an off-the-shelf RS-232-to-GPRS box. You'll probably have to build something custom -- either using a GPRS module designed to interface with an MCU (these typically have a built-in TCP/IP stack, so the MCU used can be quite basic), or a USB GPRS modem hooked up to an embedded board (my recommendation). If you're building a one-off prototype, I'd recommend getting something like an OLinuXino imx233 or a Raspberry Pi, and getting a $25 Huwei USB-GPRS modem.