What does ignorance to anything specifically "Gayatri hymn" mean?
A Brahman, ignorant of the Gayatri hymn, is more .... (Parashara Smriti,Chapter 8,Verse 24)
What does ignorance specifically mean here?
We need to see what word is used in the Sanskrit verse which is given below.
GAyatri rahito viprah sudrAdap-yasuchirbhavet |
GAyatri brahma-tattvajnAh sampujayante janair-dvijAh ||
So, the key word here is "rahita" which does not exactly mean "ignorant". It means absent,bereft of, deprived of, want or lack of etc..
So, Gayatri rahito viprah will denote a Vipra (i,e. a Brahmin) in whose life Gayatri is absent ( which can either mean that he has never been initiated into it or he is initiated but does not chant it regularly).
If a Brahmin is never initiated into Gayatri mantra then he becomes a Vratya and becomes fallen like an outcaste. And, if he is initiated but never chants it as prescribed (doing which is his Nitya Karma-daily duties) then also he is fallen.
That is what is meant in the first part of the verse.
[NOTE: The translator has translated the part "Gayatri rahito viprah" as "a Brahmin ignorant of the Gayatri hymn", which is not exactly right IMO (and as shown from the dictionary), but it does not change the overall meaning conveyed by the verse either].