The deputy president is defending himself before the lawmakers in the upper house of parliament, two-thirds of whom are required to oust him.
He faces 11 charges, including corruption, inciting ethnic divisions and undermining government.
As the trial began on Wednesday, the deputy president pleaded not guilty to each of the charges as they were read out in the house.
An overwhelming majority of MPs in the National Assembly last week voted to approve his impeachment, setting the stage for the Senate trial.
He has fallen out with President William Ruto, just two years after they were elected on a joint ticket.
The row began in June when many disgruntled Kenyans took to the streets in deadly protests over unpopular tax hikes.
Mwengi Mutuse, the lawmaker who had moved the motion, appeared in the Senate as a witness, accusing Gachagua of violating the constitution while taking the house through the various grounds of his motion.