Dear Rekha Sarma

Yes, you can increase the gain of an antenna by using FSS,

you may refer to these journals for more info.:

1-http://www.jpier.org/PIERL/pierl38/13.13021007.pdf

2-http://www.jpier.org/PIERC/pierc15/12.10070303.pdf

or , You can use an array if your design is a patch antenna. And also by increasing the height of substrate, the gain could be enhanced but the bandwidth will become smaller.

if your design is aperture(slot) coupled patch antenna, you may use Substrate integrated wave-guide (SIW) technique.

Regards 

can you please tell me about SIW technique and how can i check whether it is dual polarized antenna in HFSS

For the SIW technique, you may refer to:

http://ac.els-cdn.com/S1877705813001641/1-s2.0-S1877705813001641-main.pdf?_tid=cf551968-df15-11e5-b808-00000aab0f02&acdnat=1456772227_92918bc18e94e4010ada32639caef702

,

http://eprints.utem.edu.my/11731/1/A_New_Class_of_Dual-mode_Substrate_Integrated_Waveguide_(SIW)_Filter_with_Two_Metalized_Posts..pdf

and,

http://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=6856194

Regards 

thank you

periodic structures like FSS, metasurface can be placed as substrate below the antenna or as superstrate above the antenna for the purpose of gain enhancement.