I just went into my profile, and then at the bottom selected usage quota and clicked the storage tab. There is a purchase more storage button. I haven’t tried it, but you can try and see if it works. If you want to purchase the extra space for the group, then you need to go to a different place to buy it under the group settings. It’s all on the link I provided above.
Alternatively, if you run your own Gitlab CE installation on your own server, then you have as much space as your server provides.