Yes, the IMEI can be obtained over the GPRS network but it has to be done at the Network Operator level, not by the User at Service level.
This process is called "Automatic Device Detection" and it is applicable to a GSM, GPRS or UMTS network.
You can refer to 188.8.131.52 in 3rd Generation Partnership Project (3GPP) TS 23.012 version 9.1.0 Release 9 to have more details about this standard solution.
In few words, if this function is activated at the network operator level, the IMEI is exchanged by the user equipment to the network operator equipment (MSC/VLR/HLR) through some signalling message.
The HLR finally sends the IMEI in XML format (SOAP) to a third-party server called ADD SERVER, the protocol used between them is HTTP or HTTPS.