I want to control my RPI from my phone, so I decided that the RPI will host a site. I'm familiar with ROR and I saw that there is GPIO gem (pi piper).
Should I work with ROR or learn Django and use python's stable module instead?
You can use that gem as far as it will do whatever you want to do; it is obviously unmaintained now and doesn't include, e.g., I2C support.1 However, I notice there may be some other generic ruby modules for I2C on linux (they do not have to be specifically for the pi).
Much of the GPIO functionality can be controlled via the
/sys/class/gpio interface, which does not require any module; you are just reading from and writing to file nodes the kernel presents to userland. Basic documentation for that is here; you may find further explanations online but it is really very simple.
Note that that interface does not allow you to do certain things, such as activate internal pull-up/down resistors (but I think the
pi_piper module does). You have not said what it is you are actually trying to do, so you may or may not require that.
1. It also may not work, or work completely, on the A+/B+ or pi 2.