It is better to understand the origin of the mother Gayatri before its Yoga form in detail can be realized so that there does not lurk any doubt.
1. Particles of Earth, Water, Air and Fire are being produced by the unmanifest power or God or Brahma. Constant friction is going on irrespective of their magnitudes. The light generated in this process of friction belongs to the mother Gayatri. When this light of the mother Gayatri is generated many atomic particle start appearing in different forms and colours. These get inspired towards Divine i. e. very subtle and the physical worlds. This place or phase is the beginning of complete creation. It means that Gayatri is created by the confrontation and friction of the atoms of Tatwas automatically.
2. In the beginning of the creation the fifty seed alphabet i. e. being produced by confrontation and friction. The last Ksham denotes total destruction and is formless therefore the counting of alphabets is done only to up Ham. Therefore the form of Gayagri extends from Am to Ham and Am to Ham i. e. means Aham Brahma. It means ah : which signifies Sun (Brahma) or the sound of seed A merging into Ha. Ham represents therefore the seed for Gayatri. Therefore the first word Am (Sun) and the last seed alphabet Ham ( Sun ) are considered synonymes in Yoga. In angah A and Ha are present and A is automatically present in Ham. Therefore the seed alphabet Angah which contains the attributes of all the seed alphabets is the seed from which the mother Gayatri is born. The friction among the particles of the five Tatwas generates or is generating or had generated light and sound producing seed that is the cause of the mother Gayatri's origin. This is in fact a transformation of natural God-ly unmanifest power into a manifest form.

Aum Shantih Shantih Shantih