The club, who play at the Liberty Stadium tomorrow, finished third in the Swiss Super League last season to secure a spot in Europe, knocking out Spartak Moscow in their playoff in August.
The Swansea manager told Sky Sports News: "They don't have the big stars but they really work hard. They were underdogs against Spartak, a side who have played in the Champions League.
"Suddenly they went 3-1 up, so it shows they have a lot character, it will be a difficult game and a game where even if we're doing well we have to keep an eye on the spirit in their squad.
"They play differently than us, a little more direct at times, but they have really done well against the Russians so I want to repeat we have to be careful. We are at home, we have to try and control the game, but be aware of counter-attacks and their spirit."
Swansea beat Valencia 3-0 in their first group stage game, while St Gallen won 2-0 against Kuban Krasnodar.