How to install Gujarati fonts in iMac?
OS X already comes with Gujarati fonts, so you don't need to install anything. Can you not read this page?
If you want to type Gujarati, you need to go to system preferences/international/input sources and check the box for Gujarati, plus the box for Show Input Menu in Menu Bar, plus the box for Keyboard Viewer. Then select Gujarati from the "flag" menu at the top right of the screen and type. To see which key does what, select Keyboard Viewer from the same menu.
I recommend you do NOT install Gujarati fonts from the internet, because OS X only works right with those provided by Apple. Especially avoid non-Unicode fonts from long ago like Saumil. The international standard is Unicode now.