To be honest I cannot make out any of the characters in the calligraphy at all. But if the Romaji "Namu Jizô Ganshu Bosatsu" is correct, then it's pretty clear what it is - except for "Ganshu".'Namu Jizô Ganshu Bosatsu'
11 x 55 cm
This calligraphy, perhaps dating from Hakuin' late seventies, is a mantra, naming the Bodhisattva Jizô and calling forth his powers, Jizô is the protector of children and also leads lost souls through hell. The calligraphy itself has a powerful, almost tortuous sense of movement, and shows Hakuin's wonderful mastery of the use of ink in his later years.
https://terebess.hu/zen/hakuin/hakuin61.html
My best guess is that "Ganshu" is 願主, or "Vow Lord". That would fit with the Romaji, and it would also be appropriate for Jizo. But does anyone know if that is what the calligraphy actually says?
Here's the full Kanji (if I am correct): 南無 (namu) 地蔵 (jizo) 願主 (ganshu) 菩薩 (bosatsu).
And here is the calligraphy itself: