Guru Brahma - Guru is Brahma, who is the Lord of Creation, also called as Generator, Guru Vishnu means Guru is Vishnu (Vishnu is the Lord who is called organizer), Guru Devo Maheshwarah means Guru is the Maheshwara (Shiva or the destroyer), Guru Sakshat Parabrahma means Parbrahma viz. the supreme god or almighty.
Since Guru leads to a path of light, Guru is that Para Brahma. Tasmai Shree Guruve Namah means we bow to that Guru the guru referred to earlier.
Thus, as a realized soul, Guru is the embodiment of Para Brahma, the ultimate Godhead.