The answer is: continue to fight for the preservation of the Union
In the Gettysburg Address Abraham Lincoln’s purpose was to encourage the people, to inspire them to continue fighting as the Civil War was not over yet, he want to honored the deaths of the fallen soldiers by increasing the devotion for the cause “for which they gave the last full measure of devotion”, the way to ensure that these soldiers did not died in vain is completing “the great task remaining before” them.