Often referred to as Gettysburg-58 after later versions were published, this was the "first" Avalon Hill wargame (published the same year as Tactics II, but with product code "501" it is arguably the first). The board is a 19th century-style topographic map of the battlefield area with an overlaid grid of 1-in squares, scale: 1-in to 1/4 mile. Turns are 1 hour; units represent infantry divisions, cav brigades and independent artillery battalions (CSA) or brigades (USA) (Artillery attached to US Corps or CS Divisions is subsumed into the parent units, apparently). Counters are die-cut 1/2x1/2-in squares for HQ and zero-strength "outposts" (think pickets), 1 x 1/2 in rectangles for Cav and Arty and 1.5 x 0.5 in rectangles for Inf divisions. Units have facing and exert a rectangular zone of control to the front.
The rules resemble miniatures. In the advanced game at least, movement is free-form, the overlaid square grid being ignored for purposes of movement and combat. Add to that an optional hidden movement rule (best used with a referee) and you have a game surprisingly advanced for its era.
This version was discontinued when a hex version was published in 1961, which in turn was abandoned by Avalon Hill for another version in 1964 which superficially resembled the 1958 version, but which had significantly different rules.