Only so I think:
Its absolutely not clear to me what you want to achieve.
In your first post it looked like Gk have unit kN, in your last not!??
It is possible to let it look like in your first post by using default features
I hope you do not want to see something like 10 + 20 + 15 = 45 kN, because this would be definitely wrong!! Units out of thin air - you shouldn't do that magic production of units. Something like (10 + 20 +15) kN = 45 kN is OK, but AFAIK cannot be shown that way with Mathcad in conjunction with your one line definition and explicit symbolic evaluation.
Fortunately you can't do that in Mathcad using default formatting features. You would have to resort to some trickery like Valery's, by defining kN:=1 and later kN:=10^3*N to regain yor kiloNewtons. Nevertheless your variables would have to be redefined to get the necessary units.
Another dirty trick would involve defining a variable which looks like kN (if you don't mind the red error in the definitions) and later resubstitute it so Pk get real unit N/kN. Again you would have to redefine your Gk. All not to be recommended.
One man calculates - another checks it.
with a slide rule for example.
Probably with one of those really big slide rules. 
Although seems weird, it is possible to hide units. For the unit use different style say "User 1" and then modify the font color fo that specific style to white.
But we cannot use invisible units with/by explicit operator
And second.
An using invisible variables - one way to the Mathcad House - see http://communities.ptc.com/groups/mathcad-paradoxes
