You must specify in the roots sage theme composer.json the runtime where autoload can find the classes.
"autoload": {
"psr-4": {
"App\\": "app/",
"App\\Blocks\\": "app/Blocks/",
"Itineris\\AcfGutenblocks\\": "../../plugins/acf-gutenblocks/src/",
}
}
Don’t forget to run composer dump-autoload