Is it legal to marry a person just to gain citizenship?

I suppose it would depend on the laws in your location, but generally, this is illegal and constitutes fraud.

Its Legal to marry anyone of your choosing but to answer to your question the Dept. Of Homeland Security now requires that when you marry a person not of the US they require you both to live with each other for 3 years and they do check and ask personal questions of what each other likes and dislike are and other personal issues / so as for marry someone to obtain citizenship why would you do that anyway ? Marriage is for 2 person who are in love and that's it... and if the Dept. Of Homeland Security finds out you are getting marry just to obtain citizenship you can face prosecution