It was named Aguada de Sao Bras, by Vasco de Gama in 1497, when He set off with 170 men on an expedition to find a sea Route to India. This was in order to develop the spice route and spread Christianity. After a few miscalculations, he landed in Mossel Bay, eventually reaching Calicut in India in May 1498. A Stone Pillar surmounted by a cross, was erected by Da Gama and is the earliest navigational marker in South Africa. Mission accomplished , Da Gama returned to Portugal and was handsomely rewarded for His voyage was an extra ordinary maritime achievement, if one considered the small size of his Carvel, a replica of which is housed in the museum at Mossel Bay. Of the original 170 men only 54 returned. It was given another name; Golfo dos Vaqueiros by, Joan da Nova in 1501.  Later named Mosselbaai by the Dutch navigator, Paulus van Caerden in July 1601, this name is Dutch/ Afrikaans for mussels. As the Dutch sailors replenished much needed food stocks, with fresh mussels! Aliwal South was the alternative name, for Mossel Bay in 1850, to avoid confusion with Aliwal North, near Bloemfontein. It became a municipality in 1852. The form Mosselbaai is preffered for official purposes!
Mossel Bay is a harbour town between Fish Bay and Victoria Bay with a population of about 130,000, people. It is an important tourism and farming region of the Western Cape Province. Mossel Bay lies 400 km, East of the Country's seat of Parliament, Cape Town and 400 km West of Port Elizabeth, the largest city in the Eastern Cape Province. The older parts of the Town occupy the North-facing side of the Cape St Blaize Peninsula, whilst the newer suburbs straddle the Peninsula and have spread Eastwards, along the sandy shore of the Bay.
The town's economy relied heavily on farming, fishing and its commercial Harbour (the smallest in the Transnet Port Authority's stable of South African commercial Harbors), until the 1969 discovery of natural offshore gas fields led to the development of the gas-to-liquids refinery operated by PetroSA. Tourism is another driver of Mossel Bay's Economy.
Geolocation
22° 7' 26.4", -34° 10' 12"
References
www.history.com/this-day-in-history/vasco-da-gama-reaches-india
New Dictionary of South African Place Names by Peter E. Raper
Lighthouses of Southern Africa by Gerald Hobberman
Further Reading

http://www.petrosa.co.za/innovation_in_action/Pages/Operations-and-Refinery.aspx
https://www.mosselbay.gov.za/
https://www.tripadvisor.co.za/Attractions-g317080-Activities-Mossel_Bay_Western_Cape.html