Although, I have no experience wearing gowns, I will interject this thought.
After seeing the dress code on cruses on the down-slide, I doubt seriously if anyone would really care.
I think the most important thing is that you feel comfortable in the clothes..If your "church" clothes make you feel "dressy" then by all means.. wear them. You will notice all degrees of fashion on a formal night.
For some men just wearing a tie and a jacket is formal to them..
Enjoy your cruise.