Will this antenna even help

I think you're a bit confused with regard to the terminology here.

You link to an XBee, which is a microprocessor, connected to a RF tranciever, which has an antenna on it. You do realize you need the microprocessor and RF tranceiver, correct?

Even something as simple as sending a simple on-off signal is fairly involved. You need some sort of transmitter and receiver in addition to the antenna, you know!
Realistically, RF (radio frequency) design is hard. From your question, I think it may be a bit over your head.

There are certainly highly inexpensive RF transmitter-receiver pairs (take this tx/rx pair from Seeed studio - $4.90!). However, this will not do anything without an associated microcontroller. If you pair it with something like an Arduino, you could likely make it do what you want. However, that would involve a fair bit of work.

Realistically, assuming you just want to turn something on and off, a pre-built transmitter and receiver pair may be more along the lines of what you want.

Amtenna signals shouldn't interfere since the GSM and GPS are on different bands. However, the electronics might interfere with each other. Put GPS and GSM modules on their own ground planes such that all the parts are covered, then connect ground and supply in one place, plus bypass it with capacitors right there. Use at least three - some high quality 22pF in small package (0402), 1nF, and 100nF. At least the first two should be with NP0 dielectric. It is also good idea to bring the power from the other layer and to connect the two grounds (for GSM and GPS planes respectively) as close as possible to the input power connector (or battery).

Naturally, you want both modules as far as possible from each other, it doesnt matter whether it is on the same layer or not.

Connect to RF inputs through short microstrip. Make it thin, cover it from all sides with ground (including the other layer). Also use small high freq. parts for input filters (if its not in the module already).

Antenna performance depends on whether it is active or passive. Active ones are usualy external as they are bigger. Passive ones have low gain and it might take while for acquisition to identify enough satellites in case of GPS. The receiver correlates background noise with known pseudorandom transmission pattern of the GPS satellites since the signal is that weak. Its a wonder it works.