We prove that the 2-category Grt of Grothendieck abelian categories with
colimit preserving functors and natural transformations is a bicategory of
fractions in the sense of Pronk of the 2-category Site of linear sites with
continuous morphisms of sites and natural transformations. This result can
potentially be used to make the tensor product of Grothendieck categories from
earlier work by Lowen, Shoikhet and the author into a bi-monoidal structure on
Grt