What are two rhetorical devices Lincoln used in his "Gettysburg Address?"

Repetition:

Lincoln repeats certain key words for emphasis.  "We have come to dedicate a portion of that field. . ." "But, in a larger sense, we cannot dedicate. . ." "It is for us, the living, rather, to be dedicated here to the unfinished work which they who fought here have thus far so nobly advanced."

Read the speech for repetition of words "consecrate," and "devotion."

Parallel structure:

Parallelism has to do with sentence structure.  It is the use of the same grammatical form in a series of two or more ideas to call attention to those ideas, to create a balance in the sentence, and to create a pleasing rhythm to the sound of the sentences.

For example:

"and that government of the people, by the people, for the people, shall not perish from the earth."

In this case, the repeated grammatical element in the series of three phrases is a prepositional phrase because each phrase begins with a preposition, "of," "by," and "for" followed by the object of the preposition "the people". Certainly the effect would have been different had he said "and that a democracy shall not perish form the earth."

Take a look at parallelism here:

"The world will little note nor long remember what we say here, but it can never forget what they did here."

There is a nice balance in this two part sentence with almost identical two pieces "what we say here" and "what they did here." This is parallel structure because the same exact word order is repeated in a series of two ideas, and thanks to doing that, Lincoln establishes the desired effect--to contrast words with actions, and to also leave the most important idea for the end of the sentence, thus leaving a more lasting impression on the audience.