No, they cannot. GPS is one way, receiving timestamps via radio transmitted via multiple transmitters, then it does some fun maths involving the speed of light, and relativity. It requires the cellular link to transmit it's location to 911 via E911 services, but with the default firmware of your phone they can't remotely turn this on directly as it's not part of the E911 functionality. In order for them to turn it on remotely, they need to push a firmware patch to the handset which disables any GPS icon indications, and enables the vendor-specific command set. On top of that they have to figure out which handset is yours, which is going to be hard without an associated account with a valid GSM provider in your area. However, if they had previous knowledge of your IMEI/ESN, they could use that to locate you as IMEI/ESNs are globally unique to each GSM handset.
Also, the GPS is overkill since they can passively monitor your location via triangulation of your cellular link. This is the most likely method of monitoring, as it won't kill your battery life(tipping you off), it's passive requiring no interaction with the handset, it doesn't require the GPS chip to initialize and possibly download the GPS ephemeris if it's a cold start(which will take 40s minimum due to the 50bits/s).
Also, they could theoretically do it without a warrant if they used their _own_ equipment and knew your CDMA code - anyone can listen in to any radio transmission in the US, though decrypting a GSM/CDMA signal may be illegal. No decryption is necessary though, as long as they know your timeslot(GSM's tdm)/code(cdma).
