There exists not any absolutely free addon for installing Google Tag Manager in PrestaShop besides buying a paid addon. Prestashop also doesn’t give access to website code so we are able to add Tag Manager code but there is still an alternate way by directly modifying the website code on https://addons.prestashop.com/it/seo-indicizzazione-naturale/30904-tag-hreflang-automatici-e-tag-canonici-seo.html