How can I delete group in gitlab?

On the recent version (per posting date 11.4.4) navigate to your group's

Settings > General > Advanced > Expand

There you will find the Remove group button.

In the most recent version (Mar. 2019):

1. At top bar, click on Groups.

2. Type part of the name of the desired group or click on Explore Groups. Click on the desired group.

3. Go to Settings > General

4. Expand Path, transfer, remove

5. At the bottom of page you find Remove group. Click on the button and type the name of the group to activate Confirm button. Then click on Confirm to delete the group.