Interesting question whether or not Galileo can be used as an I2C slave. It can not, according to this Intel FAQ Intel® Galileo Boards and Kits — Frequently Asked Questions
Solution ID: CS-035127
Last Modified: 31-Dec-2014
Date Created: 08-Dec-2014
No. Intel® Galileo only supports operation as an I2C master device.
Best to find out whether that is an accurate statement before you spend more time trying to get that setup to work because you are trying to use the Gen 1 as a slave device.
I suggest you to open a new thread with your specific question because is a different topic. In there you could explain a little bit more about what you are trying to do and the configuration you are going to use, also the devices you are connecting as slave and master
It seems that you want to communicate the Galileo Gen2 and the other device using Serial Communication if the other devices requires the MAX485 then you must use it, please open a new thread so we could help you with your project.