Allowing the USSR to disintegrate,
He did not ALLOW to desintegrate, he deliberately destroyed the Soviet Union. First, STATE Media started making anti-soviet propaganda in 1987. They also allowed to do the same to Radio Liberty and many other Western organizations. Then they allowed to convert cashless ruble into cash, private international trade and something else, that lead to deficiency (latent inflation) in 1988. I've heard that they deliberately caused shortage of food by sabotage. They destroyed all the socialist allies in Europe in 1989. Finally, in 1990 the law that described the procedure to quit the Union by a republic was accepted. Gorbachev allowed Eltsin to take power, who started the process of leaving of Russia the Soviet Union.
Would you have preferred if he had never been president of the USSR at all?
В истории нет "бы". (There is no "would" in history).
What do people in other ex USSR countries think about him, for example Ukraine and Belarus?
Probably the same, but you have more opportunities to ask them than me.
What should he have done differently?
Have converted some part of military industry and gradually change stupid socialist ideology.
You haven't answered my question, Hanna.