how to bring older images from posts to galleries

Hi team,
I have a little big problem that I hope you can help me to solve.
I’ve just introduced this fantastic theme into an old web site in order to refresh it.
In it, there are thousand of posts, inside of whom are found dozens of photographs scattered in bulk and in different sizes.
However, this solution is nasty and it ruins the beautiful graphic settings of Enfold.
I would like they could be collected into a gallery so I have looked everywhere for a plugin that catch them in each post and automatically put them all into a gallery. But I can not find anything.
Since you are more experienced than me, can you tell me how / what to use for this purpose?
Thank you.