Have you tried to re-installed the USB drivers Intel® Galileo Boards and Kits — Getting Started for Windows*
If the issue continues, try to locate the driver manually.
In order to re-install the drivers, i deleted the entire arduino folder and un-zipped it again as the drivers are installed when the folder is created (I think. As last time I din't add anything for the drivers separately. ) Gadget serial still doesn't appears in the 'Other Devices' on device manager. So I tried to install it from the 'drivers' folder inside the arduino folder. There is an application there with the name of 'dpinst-amd64'. I ran it as an administrator but it says that it failed to install the drivers.(Image attached)
In any case, i think the drivers are installed from the last time as when i remove the SD card from the Galileo, it starts showing up and is working fine.
Do you think there is anything else I can try. In the mean time, I am also arranging a ftdi cable.
i have been able to solve this issue go on the sd card with the iot image there is a folder win-driver in that there is "iotdk-serial-external" folder unzip it it contains iotdk-serial-external.inf file right click on it and click on "install".Serial port will appear i followed this link :-
now it looks like this:-