Peripheral Driver Libraries on TI TM4C123 / Keil uVision
If the compiler says "use of undeclared identifier" it's because GPIO_PORTA_BASE is not declared anywhere within the various files you includes, as you guess. Three solutions:


You define it yourself.
You do not use it and give to the functions an expected parameter. To achieve this you can go to the function definition and/or prototype and see the type expected. If this type is an enumerated value (which is what I presume regarding what you are trying to do) look for the enumerated value definition (which hopefully be in the same header file) to see what parameters you can give.
Find the correct file to include that define the symbol.