I am using galaxy grand2 with stock rom(jelly bean,rooted).Day before yesterday i saw a video of modded play store on the you tube. i tried to install that but it didn't work for me. and i removed it. but my original play store is not working any more(not working in the sense it is opening but when i tried to install any app it crashes). so i want to reinstall the gapps but don't want to install the whole rom. so is it possible to reinstall just the gapps after using my phone for sometime???? thank you.
You'll need some sort of custom recovery install. If you have one click here and then select the correct version of Android you're running. Then download it and move it to your phone. After boot into your custom recovery and flash it.
Before trying to install anything open the apps section and try to get playstore in stock version(uninstall all updates) then clear cache and data of that app. If it doenst work then go to your recovery and format just system and cache, then install the ROM and gapps (don't format data or you'll lose all installed apps and files on your internal partition) Edit: you said you're running stock ROM! Stock ROM doesn't need gapps it's preinstalled in it. One workaround would be to restore the backup of your system through custom/manual restore(just the system don't check data)
I just updated the Google Play Store on my Xperia T running Android 4.3 to version 6.5.08.D-all  2792142.
You could download the Play Store .apk from somewhere (eg. here) and try to install it.