Third party assembly in custom workflow CRM 2015

I have used a third party assembly in my custom workflow. 

But I have added that third party assembly into GAC in the CRM Server.

But still I m getting this error:

Could not load file or assembly 'XXXXXXXXX, Version=5.8.606.0, Culture=neutral, PublicKeyToken=1309adb7e86da873' or one of its dependencies. The system cannot find the file specified.