Gabriel appears to have gone out in the Company's second voyage in 1604 under Sir Henry Middleton and to have been left as factor at Bantam, together with John Saris.
He arrived at Waterford in September 1613 having brought back, "Coree", the first South African to set foot in England, who was taken into the care of Sir Thomas Smythe, Governor of the EIC.
In 1617 he was again in India, apparently with some mission; Sir Thomas Roe, English ambassador at Ahmedabad to Jahangir, complained that Towerson had arrived with 'many servants, a trumpet, and more show' than he himself used.
The committee of officers appointed Towerson to command the Lesser James, on account of the differences between her pilot and master ever since they left England.
In November he was at Batavia, whence he and the other factors wrote on the 6th that, "seeing the Netherlanders are so contentious, false, and impudent in all their proceedings, not shaming to affirm or write anything that makes for their purposes, we have thought fit not to answer their protest fraught with untruths."
On 27 February 1623, the Dutch governor of Amboyna, Herman van Speult, ordered the beheading of Towerson, along with nine other Englishmen, ten Japanese and a Portuguese.
"It is true," says the official narration, "that stories do record sundry valiant and hardy enterprises of the English nation, and Holland is witness of some of them; yet no story nor legend reporteth any such hardiness either of the English or others that so few persons, so naked of all provisions and supplies, should undertake such an adventure upon such a counter party so well and abundantly fitted at all points."
Towerson married the widow of Sir William Hawkins, Mariam Khan, an Armenian Christian who was the daughter of an influential merchant in the courts of the Mughal Emperors Akbar and Jahangir.