Granddaughters are mahrams to their grandfathers, it is not permissible to marry them. They are included in the meaning of the verse:
... حرمت عليكم أمهاتكم وبناتكم
Prohibited to you [for marriage] are your mothers, your daughters ...
— Quran 4:23
Since بنات (daughters) can mean every female descendant, no matter how low i.e. granddaughter, great granddaughter, great great granddaughter and so on. Additionally there is consensus that granddaughters are forbidden for marriage. And it is also rational since Allah has forbidden nieces, whereas granddaughters are a closer relationship than them.
Ref:
— Lane's Arabic English Lexicon
وبناتكم - جمع : البنت ، فيدخل فيهن بنات الأولاد وإن سفلن
Binat is the plural of bint and it includes the daughters of one's children however far removed
— Tafsir al-Baghawy