Inbuilt flash memory size not enough
You cannot extend the program memory (flash).  TI produces the same chip with double the flash and RAM, but nothing else changed: TM4C1230D5PMI.

If you cannot use a chip with larger flash, you will have to reduce your code size:


Disable debugging, such as the expensive printf function.  A printf that supports floating point output will typically set you back around 5KB-10KB.
Make sure you compile with optimization enabled - typically the compiler flag is -Os.
Modern compilers can do link time optimization (LTO).  With gcc, you get this with -flto.  You have to pass -flto to both compilation and link stages, for all files.  This typically reduces the produced code size by 30%-50%.