There is a huge difference between The God, Incarnations and deities like Gayatri maa. Let's understand them one by one backwards.
Deities like Gayatri are nothing but the representation of some set of philosophies. Gayatri is basically the name of a 'Chhand' (type of verse form). The deity Gayatri is the sculptural or pictorial representation of the philosophies explained by the Gayatri Mantras.
When someone embodies a deity into his or her life then the person or the people are called the incarnation of that deity. This is one of the reason that most of the incarnation are the form of Vishnu. There are few incarnation of Mahakaal but as much as my limited knowledge say, there is no incarnation of Brhma. One can operate the world, one can also distruct it to some extent but none can generate. Things can be converted to other forms only. No construction or absolute destruction is possible that is why there is no incarnation of Brahma.
The God is beyond any definition that is why Hrigveda didn't even name it. I don't have the book at this time and I don't want to quote that 'Hricha' with spelling mistakes to I am just stating the meaning of it. It says about many properties of the God and then askes "Kasmai Devay Havishavidhem" (Which such deity we should worship). It also has the answer in itself "Kah asmai Devay Havishavidhem" ('Kah' such deity we should worship). In the vedic era 'Kah' was used in algebra similar to the 'X' now a days.
So! Indirectly Hrigveda says that we just know a part of the God. None can explain you about the God even if it become known.