ChemGlobe: Gallium: Electronic, thermal, and steric data along with an isotope table.
ChemicalElements.com: Gallium: Basic information, atomic structure, and isotopes.
Discovery of Gallium: Article by P.E. Lecoq de Boisbaudran.
EnvironmentalChemistry.com: Gallium: Atomic structure, chemical and physical properties, and table of nuclides.
Gallium: Data tables and historic information.
It's Elemental: Gallium: Basic physical and historical information.
LANL: Gallium: Sources, properties, uses, and costs.
Lenntech: Gallium: Physical data, chemical properties, health and environmental effects.
USGS Minerals Information: Gallium: Statistics and information on the worldwide supply, demand, and flow of the element. [PDF]
Visual Elements: Gallium: General and physical information, key isotopes, and ionisation energies.
WebElements: Gallium: Extensive information on history, uses, occurrence, compounds, and properties of the element.
Wikipedia: Gallium: Properties of the element, including its history, applications, and characteristics.