there are
- a paper by Adolf Prag in the James Gregory Tercentenary
Memorial Volume (ed. H. W. Turnbull), London, 1939, pp. 487-509.
- a dissertation by Christoph J. Scriba, James Gregorys fr"uhe
Schriften zur Infinitesimalrechnung, Giessen: Verlag des
Mathematischen Seminars, 1957 (=Mitteilungen aus dem Mathematischen
Seminar Giessen, Heft 55); the Geometriae pars universalis is
treated on pp. 29-52.
- a quite recent dissertation (and a paper in HISTORIA MATHEMATICA
if I remember correctly) by Antoni Malet (~1990); probably you will
find more information in his article "Barrow, Wallis, and the
remaking of 17th century indivisibles", CENTAURUS, vol. 39 (1997),
pp. 67-92.
Yours sincerely
Siegmund Probst