I think the joke was that the reason Forrest couldn't remember where Tex was from because Tex was really from Texas whereas all the other guys were from places totally different what they were named. That's what I think.
not sure maybe from texas but truely i don't know but their is acctually a possibilty that in real life he might live in a different state. or maybe really in texas
“ There was Dallas, from Phoenix; Cleveland - he was from Detroit; and Tex...” Lt. Dan mixes up where his solders are from. He says Forest and Buba are from Arkansas… this is probably why Forrest has peoples hometowns mixed up. But the joke is obviously that Tex was from TEXAS