Presumably you meant "propagation constant" (which is anything but
constant for real lines). In any event it is most certainly is not
dimensionless. If it were and you multiplied it by a length, your
result would have units of length. Oh, and I think you left the sqrt
off the right side of the equation. I guess Z11 is resistance per unit
length, Z22 is the radian frequency times the inductance per unit
length, etc?? Sigh.

Cheers,
Tom