If it's only a matter of adding Giga Bowser to the CSS (you could replace Bowser), then codes do exist. You can use these: CSS fixes for Giga and Company V2 [spunit262]
C2684940 00000002
9BB400B8 57BD06BE
2C1D001B 00000000
040AF8D4 546025B6
040AF964 546025B6
040AF830 546025B6
040AF528 546025B6
040AF810 546025B6
040AF880 546025B6
040AF848 546025B6
Giga and Company Engine V2 [spunit262]
C2684964 00000013
881400B8 2C00003F
41A00088 2C030010
40A20008 38600011
2C030026 40A20008
38600012 2C03001D
40A20008 3860001E
2C03001F 40A20008
38600020 2C030021
40A20008 38600022
2C03000C 40A20008
3860002C 2C030017
40A20008 3860002D
2C03000A 40A20008
3860002E 2C03000E
40A20008 3860002F
2C030000 40A20008
38600030 2C030006
40A20008 38600031
987400B8 00000000
Char ID fix 2.1
C28152E4 00000006
819E003C 907EFFFC
80E30030 2C07000F
40A2001C 80FE0008
3C008180 7C070000
4080000C 39030008
9107FFFC 00000000
And you would then need to create a CSS code with Giga Bowser in it. This would allow you to play as Giga whenever you wanted and would replace Bowser in VS. mode.
If you want Giga Bowser to show up in stuff like Classic Mode, then I have no idea.