You'll need to modify your startup_gcc.c under "(...)\hardware\lm4f\cores\lm4f\". I'll attach mine, already modified with SysTick interrupts enabled.
After that, you'll need to declare in your main program another function besides setup() and loop, called "SysTickIntHandler()". Inside that function, put the code you desire, and voilà.
//*****************************************************************************
//
// startup_gcc.c - Startup code for use with GNU tools.
//
// Copyright (c) 2009-2012 Texas Instruments Incorporated. All rights reserved.
// Software License Agreement
//
//