Gaussian produces results in units of atomic units (A.U.) or hartree.

1 hartree = 27.2113961 eV

   = 219474.63068 cm^-1

   = 627.50956 kcal/mol

All Gaussian values is in Atomic units(Hartree). For conversion, Multiply your value by 27.2114 which convert your hartree value into eV.  Usually, the band gap energy values are mentioned in eV units

I agree with Mr.Dawson. 1au =2625.5 KJ/MOL 

in some programs  the energy unite of HOMO or LUMO  is eV

in Gaussian versions the unite is Hartree [ atomic unit of energy, named after the British physicist Douglas Hartree. It is defined as 2R∞hc, where R∞ is the Rydberg constant, h is the Planck constant and c is the speed of light] , the relation between  Hartree and  eV  units :

1 Hartree = 27.2113961 eV 

May I just point out that legit unit of measur for energy should be dimensionally speaking energies?

Can anybody recommend any references to support the number of 27.2114?

Dear Hao Wang

Reference: K. I. Ramachandran, Gopakumar Deepa, Krishnan Namboori, "Computational Chemistry and Molecular Modeling: Principles and Applications", Springer Science & Business Media, 2008, ch. 1.5.1, page 5.

The number follows from the definition:

Eh = h^2/(4·pi^2·me·a0^2) = e^2 / (4·pi·e0·a0)

where h = Planck constant, me = electron mass, a0 = Bohr radius.

See also:

https://physics.nist.gov/cgi-bin/cuu/Value?hr

https://physics.nist.gov/cgi-bin/cuu/Value?eqhrev

Dear all,while converting homo- lumo energy in ev multiply it by 27.2114 then divide these value by 1240 it gives the particular wavelength .