Robin's reasons for not applying for a GRC are set out here, in evidence given to the Women and Equalities Committee in 2020.
committees.parliament.uk/writtenevidence/16982/pdf/
Interestingly, in addition to saying "It does not appear to me to determine any rights which are significant to me", Robin appears to be against the secrecy clause in the GRA: "I do not like the fact that, if I obtained a GRC I might be the cause of, for example, and HR adviser collecting a criminal record for a mistake
in connection with revealing my history under s22 of the GRA."
So Robin seems to think that (1) GRC's are completely pointless and (2) they can lead to people who make an innocent mistake being prosecuted and acquiring a criminal record.
With this evidence in mind, I'm somewhat mystified as to why Robin thinks that changing the GRA to a self-ID process and allowing anyone to change their legal sex on that basis is a good idea.
(I haven't read through the whole document, just that section on why Robin doesn't have a GRC, so maybe this apparent contradiction is explained elsewhere.)