the second with 'gonna' is not English, street or lazy language, being the same as the first. I will marry you is DEFINITE, speaks of determination. I shall marry you is less emphatic. Will and Shall are slightly different. I am about to marry you is a little strange and artificial, because usually it is two people who are about to be married. About to means imminently, soon. You'd more likely hear 'We are about to get married'