Hi Frank,
You can use Showtime Image Gallery in several ways in a Joomla site:
1. As a custom field.
2. Inside article text using the editor button. For extensions with no editor you can use the following code:
{showtime X}
. Just replace the X with the gallery ID you want to display.
3. Using a module.
Not sure which of those methods work for SP Pagebuilder. If you go for the module and want to use a specific gallery then just assign that gallery to one category and configure the module to fetch galleries from the same category.
Please let me know if you need more help with this.
Regards