[gokaygurcan]

As a company, we use Atlassian Stack + Skype like most others do. So, Jira (issue tracker), Confluence (documentation), Stash/Bitbucket (git + review), Bamboo (ci), etc.

Personally, I also have Jira, Confluence and Bitbucket licenses but I mostly use GitHub's issue and wiki pages and Travis.

Since you're using GitLab already, I think it's best to stay in there. It has a nice issue tracker and issue board as well as other things you might need.