This may help someone having similar problem.
I had gerber files generated in CST MWS (RF designs) but wanted to make a panel for fabrication using Altium Designer. Altium easily generates GBR from PCB document but panelizing all GBR's into one GBR is a little different from what is available in their website (

Note: These are single sided designs to be printed.

Import first Gerber file into a new CAM document. In Tables-> Layers, assign Top layer.
Go to Tools-> Panelize PCB. Make a 3x3 or 4x5, whichever size you like.
In the Tools->Preferences -> Goto Import/Export and select ‘Use Existing Layer’ option at the bottom. So all the next imports automatically come into Top layer.
Import more designs and Edit-> Move to the boxes generated by panelizing PCB and keep adding all the gerbers.

In the end , File-> Export-> Gerber.