Why do i install dlcs individually?

Can't i mark dlcs that i want to install(or delete) and install(or delete) them together?

Please check this function.

ps. If this function is already exist let me know please.

Since Galaxy is still in beta it has quite a few basic features missing as it only just got the option to pause downloads fairly recently. For more details regarding that you can check the following thread.

http://www.gog.com/forum/general/gog_galaxy_beta_2

As for DLC installation you can vote on the wishlist for this feature to be implemented in a later version and create any other feature request that isn't already there.

http://www.gog.com/wishlist/galaxy/add_download_all_dlcs_button