We have already setup done, hosted Git repo's through Gerrit and Fisheye used for indexing & code browse with Crucible as review tool.
Now, oraganisation planned to move to Stash because of it's extensive features. I need to know the best way to migrate all the Git repo's from Gerrit and Fisheye to Stash. Is any tool need to be use to achieve this? or is Atlassian have any plugin to complete this.
It really depends on how many repositories you have. Being a distributed version control, with Git you can simply clone your repository locally from Gerrit/Fisheye, and then push back to Stash. The fact that you're migrating from a non-Atlassian tool to an Atlassian tool shouldn't matter.
If you have more than 10-20 repositories this might become a little tedious, at which point I would probably write a script that reads all your repositories from either Gerrit or Fisheye, creates corresponding repositories in Stash and does the clone/push automatically.