Most phones can be used by embedded micros to send text messages. Those phones which allow you to communicate via an RS232 serial port basically present a standard "AT command" modem interface. This means that with a micro and a voltage level converter (the ubiquitous MAX232 MAX232) you just send a series of characters out of the serial port and the phone does all the difficult stuff. If the phone doesn't have any interface like that, you can hack into the keypad and use something like the CD4066 switches to fake key presses. It is slower but it still works.

Similarly, many GPS units can be driven down a serial port. You just give it an initial set of info, and the module spits out a set of data every second (or less if programmed). It isn't difficult to process this data into a more human readable form. Loads of info is obtainable around the net for both of these, it purely depends on which phone / gps you go for.

Just curious how your project is coming along ... hope you're having some fun with it.