Sorry @MorganB. Due to cost saving considerations GigaRocket doesn't have the premium paid version of Softaculous. So you will need to install myBB manually. Which shouldn't be difficult to do though, particularly if you do it with the cPanel File Manager. The cPanel File Manager is really very good.
If my steps below are too elementary, hope this does not offend. This is usually how I do this. Before you start this can you check your php version in your cpanel, as possibly you may want to have a specific version first. You can check cpanel for changing the version.
2. Then unzip the files to your computer. Select the portion of the files that are going to be directly under your domain and rezip those, i.e. don't include the main folder myBB, only the files under myBB so they will appear directly under your domain for mybb.
3. Navigate to File Manager in Cpanel. Then navigate to the root folder of your domain. Then use the upload tool to upload the zipped folder from your computer to your hosting account so it's located directly under your mybb domain.
4. Unzip the contents of the folder
5. Visit your domain with your browser and it should then automatically come up with the installation page that starts the installation of myBB.
6. If everything is OK - delete the zipped folder in File Manager