The battle would be ridiculously one-sided.
* Inside Boo, Goku is forced to use his SSJ against Mystic Gohan. In GT, Base Goku had the upper-hand against Base Rilldo, who is stated to be strongest than Boo. Even assuming the original Boo (so, his true, own strength), this form is stated to be the strongest, thrice. Base Goku (GT) > Base Goku (Inside Boo). Baby Vegeta > SSJ3 Goku (GT) > SSJ3 Goku (Boo)
To further confirms Rildo's strength, Gohan was stated to have trained in GT, based on the fact that he was able to go SSJ. It might implies that Base Gohan still retained his Mystic power and using his SSJ on top of that thanks to his training. But he got stomped by Rildo.
* Inside Boo, Goku is forced to use his SSJ against Mystic Gohan. In GT, Base Goku had the upper-hand against Base Rilldo, who is stated to be strongest than Boo. Even assuming the original Boo (so, his true, own strength), this form is stated to be the strongest, thrice. Base Goku (GT) > Base Goku (Inside Boo). Baby Vegeta > SSJ3 Goku (GT) > SSJ3 Goku (Boo)
To further confirms Rildo's strength, Gohan was stated to have trained in GT, based on the fact that he was able to go SSJ. It might implies that Base Gohan still retained his Mystic power and using his SSJ on top of that thanks to his training. But he got stomped by Rildo.