There is a book called Vaitheekasri, which gives lot of information. It is
a monthly magazine being published by Shri Nannilam Rajagopala Ganapadigal.
It contains very useful information including about all festivals and about
Sankalpams for monthly as well as for Amavasya Tharpanam. There is
mention about the Gayathri in the August month magazine. Wherein Maha
Periyavaa in his discourse has clearly explained the need for Gayatri Japam
necessity for doing Sandhyavandanam. The same is reproduced here for
information.
Quote :
If the Gayatri has not been chanted for three generations in the family
of a Brahmin, its members lose caste (they cease to be Brahmins).
The quarter where such Brahmins live cannot be called an "agrahara".
It is perhaps not yet three generations since Brahmins gave up the Gayatri.
So they still may be called Brahmins.
In the same way if the Brahmin family has not performed sacrifices for
three generations its members will be called "Durbrahmanas", degenerate
Brahmins. Even though degenerate the label "Brahmin" sticks to them.
There are prayascittas (expiatory rites) by means of which the
corrupted Brahmins will be remade true Brahmins. But there is no such
hope for a Brahmin in whose family Gayatri has not been chanted for
three generations. A member of such a family ceases altogether to be
a Brahmin and cannot be made one again. He is just a "Brahmana- bandhu",
a kin or a friend of Brahmins. The same rule applies to Ksatriyas and Vaisyas
with regard to the Gayatri mantra; they become "ksatriya-bandhus" and
"Vaisya-bandhus" respectively.
You can also read this in the website :
Kanchi Periva Forum
Balasubramanian