> Intercept is like a one-sample t-test? It is the total group effect? basically do my network exist on a group basis?

Yes, you can add an option

-gltCode 'gender : 0.5*M +0.5*F'

to obtain the directionality of the population-level effect.

> - I guess that my contrast of interest is 'gender:age F'?? However, I do not have an equal distribution between male (n=6)
> and female (n=2) is it a problem? Because it seems that I have way more contrast on 'age F' alone that on 'gender:age F'.
> Even if the contrast for 'gender F' is weak.

The issue is not really about the unbalanced situation between the two genders, but about the meager number of samples in each gender. You may consider more specific inferences such as

-gltCode 'gender : 0.5*M +0.5*F age :'
-gltCode 'gender : 1*M -1*F age :'
-gltCode 'gender : 1*M age :'
-gltCode 'gender : 1*F age :'

> Finally, if I want to convert the F stat into z values the command would be?

Don't do the F-to-Z conversion. Instead, rely on those post hoc inferences listed above.

Gang