My guess is the casinos in Nevada and Atlantic City which were perfectly legitimte businesses. Of course in real life I do not think Atlantic City ever made tons of money, but I am not sure.

I think he "sold all the casinos" right before he spoke with the archbishop. Assuming he bought in Nevada in the 50's, and
expanded his operations by moving Klingman and Roth out, by 1977 the value of them would have been significant, especially if he sold to huge corporations as really happened in Vegas.
This would also explain Michael paying back his investors at the meeting in Atlantic City just before the chopper attack.