Gideon's knowledge of the drunkenness of the Lamanites tells us that Gideon has had spies in the field. He would need to know the state of the various Lamanite encampments.
Mesoamerican cultures appear to have had a long-time difficulty with the excesses of alcohol. Jacques Soustelle analyzed the later Mexica laws concerning drunkenness:
There were ferocious laws against public drunkenness. The statutes of Nezaualcoyotl punished the priest taken in drunkenness with death; and death was the punishment for the drunken dignitary, official or ambassador if he were found in the palace: the dignitary who had got drunk without scandal was still punished, but only by the loss of his office and his titles. The drunken plebeian got off the first time with no more than having his head shaved in public, while the crowd jeered at him; but the backslider was punished with death, as the nobles were for their first offence.
Here we have an exceedingly violent case of socially defensive reaction against an equally violent tendency, whose existence has been historically proved, for when the conquest had destroyed the moral and judicial underpinning of Mexican civilisation, alcoholism spread among the Indians to an extraordinary degree." (Soustelle, Jacques. Daily Life of the Aztecs on the Eve of the Spanish Conquest. Stanford University Press, 1962, p. 156-7).
While this example comes from much later than the Book of Mormon, and an entirely different culture, there is no reason to suppose that the potential problem with alcohol was much less in earlier times and different cultures. As Soustelle noted, when the severe cultural penalties were withdrawn, the problem of alcoholism became widespread. In the current case we have some Lamanites who are guarding the back gate who are drunk every night. Gideon does not mention any other group of guards.
A possible picture of the situation would be that the Lamanites also had a fairly rigorous injunction against drunkenness. This particular guard party, however, appears to be isolated away from other encampments. Those encampments closer to Shemlon, and with greater contact with that city, would probably have been more circumspect. However, this guard party by the back gate might not expect any particular problem. Gideon speaks of a secret pass (verse 7). This would indicate that the area would have been generally considered impassible (except perhaps through the particular location guarded by the Lamanites).
When combined with the Lamanite drunkenness at night, all of this suggests that the Lamanites thought that there would be little threat, and that the night would be even less of a problem. This Lamanite sense of complacency would be increased by the lack of probable trouble as well as the lack of recent troubles. When combined with their relative isolation, it would appear that they literally let their guard down.