None: Timers in TM4C123G not working at all

I wanted to use timers in TM4C123GH6PM : http://www.kynix.com/Parts/178828/TM4C123GH6PMIR.html(in TIVA C series TM4C123G Launchpad evaluation kit). So I decided to use GPTM TimerA0 in periodic timer mode. I defined the address of the registers of GPTM and followed the steps given in the section 11.4.1 One-Shot/Periodic Timer Mode of the datasheet in page 722. I want a LED to blink after every 3 seconds (connected to PORT F-pin 1). But the LED is always ON. Are the address of the register