For an initial introduction to GAMS you may consider reading either Richard Rosenthal's tutorial or the Quick Start Tutorial of the Bruce McCarl Expanded GAMS User Guide, which is included in the GAMS IDE: Help → Expanded GAMS Guide (McCarl)

For model examples you can browse our Model Library at: http://www.gams.com/modlib/modlib.htm which is also available as part of any GAMS system. Within the GAMSIDE just click on: File → Model Library → Open GAMS Model Library