I am not up to that speed in RoR to make it. I would just
prefer a solution in RoR that I know how to at least tamper with

I know PHP quite well but still any project in PHP that size looks
like spaghetti to me
You can make spaghetti in Ruby and in RoR as well, but it never quite
looks like spaghetti as much as PHP does (even when it isn't)
I would say, though, that you'll be well off picking up the Sitepoint
book on Rails. Great intro to get you up to speed.
It doesn't throw the kitchen sink at you quite the way AWDWR does
(but AWDWR is still a vital reference)
While using it, develop your own gallery at the same time. Just add
the features you know you will need.
For each feature, if it seems terribly complex, try building a new
app that is only focused on that feature, to work out your logic.
Seems like overkill? Ruby and RoR make it pretty easy to do that.
Once you get over the first learning curve of RoR, everything else
comes a lot easier and it becomes a matter of looking up other
features of the framework.
Surely you don't need ALL of the features in Gallery2?! It tries to
be everything to everyone and in doing that, it ends up with a lot of
things you might not want/need.
Building something like this for yourself is the way to get up to speed!