Try the suggestions in this thread
Adafruit SSD1306 [SOLVED] - #8 by ScruffR
Although this mainly deals with SPI the code alterations are still valid for I2C too.
Just use the Web IDE library with the example - no need to add file tabs and copy-paste the lib code (unless you need to alter the lib for other reasons).
Don't import Adafruit_GFX library as it is already present in the SSD library (and in your project as seperate project files).
This is most likely the reason for all these
... multiple definition of ...
BTW: This is a bit ambigous
There are two Particle IDEs - Particle Build (aka Web IDE) and Particle Dev
Which one are you using?
The suggestions above apply to Particle Build, which I'd recommend for starters.