Hi,
You need to specify a substrate in order to generate the gerber file. Please see the following code.
ant = spiralArchimedean('Turns',6, ...
'InnerRadius',12e-4, ...
'OuterRadius',975e-4, ...
'Tilt',0, ...
'TiltAxis',[0 0 1]);
r = reflector('Exciter',ant,'Substrate',dielectric('fr4'))
r.Spacing = 0.025;
p = pcbStack(r);
p.gerberWrite