As a teacher of another Arts subject, I find it abhorrent when schools do this. It effectively kills off the subject because there will be no students coming through to A Level if there's no GCSE. Arts subjects are always going to have smaller intakes and schools know this and should budget well in advance for it. Not offering them at GCSE should never be allowed to be an option. It basically says loud and clear to the student body - the Arts don't matter.
Students who want to pursue the Arts at A Level and university can struggle without the grounding of GCSE. With Drama/Dance/Music it's easier to do A Level without any prerequisites if a student has been doing LAMDA/ABRSM/RAD etc outside of school, but Art doesn't have this strong extracurricular tradition and has a lot of theoretical and technical teaching at GCSE that most kids aren't going to be getting elsewhere.
The school has really done the dirty on you here by letting you know they're dropping it at the last minute. I would not accept this.
I know it's hard at secondary with parent contacts, but could you find out who else signed up for Art and still wants to do it, and club together as a group of parents to make a complaint and force the school to find a solution? If there are other subjects with smaller numbers that are still running at GCSE, and there is an Art teacher available to teach it, then their decision to scrap Art as opposed to say, Geography, is discriminatory.
If they refuse to offer the GCSE as a taught subject in school after you complain, three other possible avenues are as follows -
a) those students wanting to do GCSE Art do it as an online course and are given dedicated time in their timetable to be in the art room to do this work, supervised by a technician
b) the school arranges for the students to take the GCSE at another local school (this is quite commonly done at A Level, not so much at GCSE, but if there is another school within short walking distance offering GCSE Art, this could be possible)
c) the school allows your daughter (and any one else who wants to do it) to drop one GCSE at school, and you enrol your daughter in an evening class to do GCSE Art elsewhere. This would be challenging workload wise but not impossible - if she had a couple of free periods at school, she could use those for homework that would then buy her back time in the evening to attend the Art class elsewhere.