This is a script capital G or, in TeX notation, {\cal G}. It reflects the use of multiple styles of the same underlying alhabet in mathematics and sciences.
It is not a capital script g (note the different ordering of capital and script).
--Jörg Knappen
I had found in 2013 a GꞬ contrast in mathematical notations of an old (1952) physics book (see http://www.unicode.org/mail-arch/unicode-ml/y2013-m01/0092.html)
Frédéric
Frédéric
