UNCLE Walter wouldn't recognize his old stamping grounds.The once-peaceful Gettysburg torn apart in July of 1863 by three days of North-South carnage is a peaceful little town again. But instead of depending on farming, as it did when Uncle Walter and 160,000 other soldiers came to call, it now lives off the tourists who have been coming here almost since the last shot was fired. More than one million visitors come every year; more than 4,000 every summer's day and still more during the July 1-3 anniversary of the battle that was one turning point of the Civil War. While the throngs of tourists march over the core battle area of the town, a relative handful of blue and gray troops stroll the sidewalks in two and threes: A blue-clad "Union officer," complete with saber, strolls downtown; two enlisted men in dark blue emerge from a fast-food restaurant, and a gray-clad reenactor, playing swashbuckling Confederate Gen. Jeb Stuart, sets up tent headquarters where the left wing of Major Gen. George Edward Pickett's charge would have swept through on the battle's final d ay. A ranger about to begin a lecture asks tourists where they are from: Hawaii, Connecticut, Washington, Canada, Ohio, New Jersey, shoot back the crowd. When Uncle Walter was here the 160,00 uniformed troops of both sides far outnumbered the mere 2,600 civilian residents. It was 128 years ago today that Uncle Walter - then Private Walter Winward of the 16th Massachusetts infantry regiment - marched into Gettysburg to find himself in yet another major battle. Within 12 hours he and his fellow Bay Staters were huddled in a narrow line, part of the dangerously exposed left side of the Union position. Three hours they waited in the eery quiet before Confederates unleashed a horrific attack from across an open field. At day's end the 16th Massachusetts and several other regiments were in tatters, neither side had gained appreciable ground, and the stage had been set for the final day's climactic frontal Confederate assault - Pickett's charge. What a difference 128 years makes. The killing fields of 1863 have become the hay fields of 1991, and it is harvest time again. The great field across which Uncle Walter stared apprehensively is now benign tan stubble: Behemoth mowers have just swept it clean of hay. In the woods beyond, where Longstreet and fellow officers barked attack orders, the sharpest command today comes from a four-year-old boy: "Stay on the path! Momma says don't walk on the grass." Less than a mile away 200 Confederate artillery pieces once fired in ear-splitting din to soften up the Union lines before Pickett's charge; today the only sounds are the chirping of a sassy blue jay, and the squeak of bicycle brakes as two helmeted tourists slow to drink in the scene. Harvest hay is being rolled into cylindrical bales along the gentle slope in front of the Union center where Pickett's 12,000 men were cut to ribbons while the remnants of Uncle Walter's regiment waited in reserve. More than hay is being harvested nowadays. By far the biggest harvest is knowledge, and the harvesters are the tourists. Scoutmaster Robert Donley brought 25 Boy Scouts to the battlefield park from Waynesbrook, Pa. Says Mr. Donley: "It is very important for the scouts to understand ... how important a landmark Gettysburg is." Uncle Walter would agree.