Hi Maryam,
As of the DSpace 7.6.1 release this should be working out-of-the-box. In fact, in DSpace 7.6 we had complaints it was recording **too many** downloads, as thumbnail downloads were being recorded. See
https://github.com/DSpace/DSpace/issues/8938 That issue was fixed in 7.6.1 however.
As noted in that issue ticket 8938, I believe downloads in Google Analytics are recorded as "item" events with the action "download" and category "bitstream".
If you are not seeing those events in your GA, then I'd recommend looking at your logs (dspace.log) to see if errors are being thrown every 5 minutes (as the default "google.analytics.cron" will send these download stats to Google Analytics every 5 minutes).
Tim