While living in the Dutch Republic, Gmelin developed a keen interest in marine algae.

Whilst travelling in the Caucasus he was taken hostage by Usmey Khan of Khaïtakes and died of ill treatment in captivity in Akhmedkent, Dagestan.

Gmelin was the author of Historia Fucorum (1768), the first work dedicated to marine biology dealing exclusively with algae and the first using the binomial system of nomenclature.

However, algal specimens used by Gmelin in the Historia fucorum are thought to no longer exist (Dixon & Irvine, 1970).

The results of his travels were published in Reise durch Russland zur untersuchung der drey natur-reiche ("Trip Through Russia To Study the Three Natural Provinces") (1770–1784, 4 vols.).