Can hermaphroditic horsetail gametophytes fertilize themselves?

Yes, self-fertilization of hermaphroditic horsetail gametophytes can occur.1

In Equisetum species, spores produce two types of gametophytes: smaller ones that only develop male antheridia, and larger, hermaphroditic ones that produce female archegonia first and antheridia later. Because these hermaphroditic gametophytes possess both types of sex organs, they are capable of self-fertilization, which can result in the production of several sporophytes on a single gametophyte.1