Making my own Breakout board for a GPS
Is the ground plane nessecary?


Yes, only to control the characteristic impedance of the antenna trace. Without a ground plane it is very difficult to ensure 50&ohm;. For such a short trace, you may get away with it, but you might find that depending on where you place your prototyping board, or how close you hand comes to it, has a big effect on the reception performance of the antenna.

To save yourself the trouble of adding a ground plane, for the sake of prototyping I'd just move the connector closer to the pin and live with the possible degradation to reception.


  Should I put decoupling caps on the PCB or leave them to the prototype?


Put them on. The inductance of the pin headers will make external caps pretty useless.


  What should I change to acomodate to the 5GHz frequency /antenna?


Make that antenna trace as short as possible.


  How can I put power protection on this aside from a fuse?


I don't see power protection as necessary. You're likely to kill the chip due to over voltage before you manage to blow a fuse. If you're concerned about that, add some Zeners and a fuse.


  Is this a viable setup for prototyping?


Don't see why not.

PS. That gnd traces on pins 18 and 20 look painfully thin.

PPS. Nice schematic symbol for the antenna!