Metamaterials as you know are artificially constructed composite materials having properties like negative permittivity, negative permeability or both. Due to negative refractive index (when negative permittivity and permeability lies in same frequency region) the propagation of waves in such a medium is governed by Left hand rule and the direction of propagation is reversed(it seems to be coming from sink towards the source).

Similarly, When you see working of transmission lines, they carry energy in a particular direction which is usually the direction of propagation. If you want to observe an effect similar to what is seen in negative refractive index material with transmission lines, then although you can not have a pure left handed transmission line(only backward transmission from sink to source), but you can see backward propagation along with forward propagation in CRLH transmission lines.

So you may say that CRLH or metamaterial based transmission lines are another type of metamaterials.

So, can I still have transmission line metamaterial behind the antenna? Also, doing still need to have gap between the antenna and the transmission line metamaterial? Can I still expect unidirectional radiation pattern ?

You can't use transmission line metamaterials as metamaterial covers but You can use CRLH transmission line based antennas, i.e. CRLH transmission lines can be used to radiate like antennas (just like leaky wave antennas)

Oh. The trouble I am having is to design a metamaterial from 2 to 6 GHz and also has less thickness and less gap in between the antenna and the metamaterial. Any advice is much appreciated

I already replied to your message. Kindly check it there