Both gentle fist and elemental jutsus are useless to kaguya considering her arsenal, and she actually did use TSB.
You're confusing yourself, kaguya's ash bones and kimimaro's bone tech are completely different in terms of creation,manipulation and of course result.
Gentle Fist is one of the strongest techniques in the right hands, if she were capable of using it Naruto and Sasuke wouldn't have been able to use chakra and hence wouldn't have sealed her.
Kaguya's Ash Bone is just the bone tech at it's fullest potential, likewise her TSB are elements at their fullest potential, her dimension shift is Ameno at its fullest potential and her chakra fists were Byakugan taijutsu at fullest potential.
She only knows how to use her KKG in their raw form, she isn't able to focus her power to use specific techniques which Ootsutsuki developed over time which is why she never used Kimimaro's techniques.
Kimimaro can also form bones out of his body like her so his Bone jutsu is still Kaguya's ability regardless of whether she can or can't use it, her Ash Bone technique would have been what he unlocked if he were in possession of Hamura's Rikudou chakra.