Traditional pharmicist's abbreviations for a drop. Originally, gt was the singular (1 gt) and gtt the plural. The symbol comes from the Latin word gutta for a drop
Russ Rowlett (Director), How Many? A Dictionary of Units of Measurement, Center for Mathematics and Science Education University of North Carolina, USA 2005. http://www.unc.edu/~rowlett/units/dictA.html, (Unit of Meas and Covr Factors - Amer) {608}