hello , can anyone tell me can i connect / use GPS system with kk 2.1 board. if yes than which system should i use. and from where can i buy this cheep...

no! kk2.1 is not an autonomous system, and merely a multirotor aircraft stabilisation unit. there are other systems like multiwii, apm, pixahawk, etc which are autonomous systems which interact with gps, and other navigational sensors for the job.

but i heard somewhere that you can connect kk2.1.5 with GPS

you need a custom firmware to do it, not completely impossible, but the options are not that fantastic compared to the tried and tested ones..

ok thanx

Well just in case, you can find some more info here with custom firmware(modded multiiwii actually) and connections

http://www.rcgroups.com/forums/showthread.php?t=2157987