You can use CloudLinux Selector to enable GD extension in PHP via DirectAdmin.
1. Log-in to your DirectAdmin account.
2. In the Extra Features section, click on the Select PHP version icon or type Select PHP in the navigation filter box. The option will appear. Click on it.
3. Click on the Extensions menu.
Note: If the Extensions page shows an error related to the native PHP version, change it to a non-native version first.
4. Tick mark the GD Once enabled, it will display a success message.
The message will read that the module GD is saved.