At my company, we use Gitlab only as version control system and it works very well for around 100 programmers. We run it on an EC2 medium instance, around 200 GB standard SSD storage.
We don't use Gitlab for CI. We use Jenkins.
For task and issues management we use JIRA.
Yes, it's 3 tools to manage but since each of the tools is really optimized for its task, it works really well.