How is impedance matching for chip antennas practically done?
First I would second the old talk to the manufacturer first plan, but then I would say you come up with an idea based on what you know about your radio in/out impedance and your antenna input impedance considering that your goals are likely:


to efficiently match your receiver and/or transmitter output to your antenna impedance (smith charts anyone?)
DC blocking
acting as a balumn between the transceiver output and the antenna
suppressing any harmonics that will get you in trouble when you go for certification


Although in your case I guess you'll be receive only for the GPS portion.

After that I'd simulate it in a tool like CST microwave studio here's a nice appnote from them on matching networks and another here.   The faster you go the more interesting parasitics and the layout itself become so you can only get a real answer by simulating or testing.

Usually you want to simulate first to get an idea of how things are going to work.  Then you build a bunch of variants for lab testing and for transmitter testing you use something like a spectrum analyzer (or RSA) with a reference antenna to get an idea for how your whole circuit performs. To test receiver side you'll need some kind of known source, I've never tested GPS but there are plenty of fancy tools out there like this Spirent simulator and others I'm sure.  You might get away with just a signal source and a known antenna depending on what testing capabilities are built into your chipset.

Depending on what kind of gear you have and your level of commitment you might also consider building some test circuits and using a Network Analyzer to understand the impedance of your circuit.

Hope some of that is helpful, I'm not an RF engineer by any means, so someone else may have come along with a better method.