I've now read the Guardian article and the previous Resolution it refers to, and assuming the article is accurate, it's clear the red line here is abortion provision, even if it may also the case that the USA want "sex-based violence" to replace "gender-based violence".
Article says:
In cases of disagreement in the security council, member states often fall back on previously agreed text, but the US has made it clear it would no longer accept language from a 2013 resolution on sexual violence.
“They are threatening to use their veto over this agreed language on comprehensive healthcare services including sexual and reproductive health. The language is being maintained for the time being and we’ll see over the next 24 hours how the situation evolves,” Patten said.
“It will be a huge contradiction that you are talking about a survivor-centered approach and you do not have language on sexual and reproductive healthcare services, which is for me the most critical.”
In a draft of the resolution seen by the Guardian, the contentious phrase is only mentioned once, in a clause that “urges United Nations entities and donors to provide non-discriminatory and comprehensive health services, including sexual and reproductive health, psychosocial, legal and livelihood support and other multi-sectoral services for survivors of sexual violence, taking into account the specific needs of persons with disabilities.”
This is that Resolution 2106 (2013): undocs.org/en/S/RES/2106(2013)
The term "gender-based violence" appears all through the report, together with "sexual violence". If the USA is focussing on that one paragraph, then this is about abortion.