How to add theme from github with composer
If you still want download theme from github, you can edit file composer.json by add more repository: 

Add this code

 {
    "type": "package",
    "package": {
        "name": "forumone/gesso",
        "version": "2.0",
        "type":"drupal-theme",
        "source": {
            "url": "https://github.com/forumone/gesso.git",
            "type": "git",
            "reference": "8.x-2.x"
        }
    }
}


after 

{
    "type": "composer",
    "url": "https://packages.drupal.org/8"
},


and run composer require "forumone/gesso"
 to download theme and update composer.lock.