According to Joseph Allen (River Grijalva = Sidon River), the "valley of Gideon" (Alma 2:20) may have been the beautiful valley area located along the Chiapas mountains between Comitan and San Cristobal de las Casas, both modern cities in the State of Chiapas, Mexico. This pleasant valley rises to about 6,000 feet above sea level. It is located east of the Central Depression area of Chiapas. Hence, it is east of the River Grijalva (Sidon). The nearby town is called Villa Las Rosas. It is about 112 miles from the Mexican-Guatemalan border. [Joseph L. Allen, Exploring the Lands of the Book of Mormon, pp. 355,380]
“Valley of Gideon”
The Nephites probably pursued the Amlicites from the hill Amnihu to the valley of Gideon in one day (Alma 2:19); however, during this time of battle over 19,000 soldiers lost their lives.
According to John Sorenson, they traveled uphill, for they went "in[to]" the valley, so the pace and distance was less than expectable for normal battle travel, or about 20 miles [John Sorenson, The Geography of Book of Mormon Events: A Source Book, p. 231]
Note* As far as the direction, it might have been somewhat "southeast" for the following reason. If the "valley of Gideon" Alma 2:20) was named after the Gideon that was slain (being possibly the place where he first settled after coming from the land of Lehi-Nephi), and this valley was close to the land of Minon (within a night's travel and return) and the land of Minon was "in the course of the land of Nephi" (in the path towards the land of Nephi), then this might also put the location of the valley of Gideon somewhat in the direction, route, or pathway towards Manti which was "by the head of the river Sidon". Notice, however, that the land of Manti is not mentioned until later on in Alma 16:6. [Alan C. Miner, Personal Notes] [See the commentary on Alma 2:25-27; 3:20; 6:7] [See Geographical Theory Maps]