try using cocn H2SO4 98% (cheap and comercially availible just add through a droppiong funnel slowly!! ) this should speed this up significantly or heat it until reflux

check this refrence out to form it from the chloride  DOI: 10.1021/ja01286a011

The Precipitation of Basic Gallium Sulfate by Means of Urea. I. A Study of the Precipitate Produced Hobart H. Willard, H. C. Fogg J. Am. Chem. Soc., 1937, 59 (7), pp 1197–1199

or alternatevely disolve gallium chloride in toluene or hexane and add conc H2SO4 ( this liberates HCL gas)  and should form the sulfate

the sulfate is also comercially available from sigma aldrich

hope these help

J

http://pubs.acs.org/doi/abs/10.1021/ja01286a011?journalCode=jacsat