SIM908 GSM and GPS modules individual power control
[This started as a comment, but I ran out of room.]

Having the GPS on while GSM is off (or in a low power mode) may be possible.  Look into the hardware design guide for SIM908 (4.2MB .doc).  Section 4.3 describes the low power modes for GSM and how to enter them.  Section 5.2 describes power control for the GPS.

Somebody who had first-hand experience with this module might be able to provide a more definitive answer.  E-mailing a carefully crafted question to Simcom 's application support might also be a good idea.  (And if they tell you the answer, make sure to post it here on EE.SE for posterity.)