Yes, the Galileo can boot from an SD card. It’s recommended you use one of the images provided in the software downloads website IoT - Intel® Galileo Board Download | Intel® Developer Zone .
Using one of the images on the SD card will allow you to boot using the grub. You can see more information here IoT - Step 1: Make a bootable micro SD card | Intel® Developer Zone
If you use one of the provided Linux images you can access the gpios through the Linux console. There is a variety of options to configure the various gpios. You’ll find more information in the directory /sys/class/gpio/.
Take a look at this guide EmutexLabs . It is a very complete guide that describes most of the general aspects of the board very clearly.