The Canadian women also skated to silver in the relay.
St. Gelais, from St. Felicien, Que., lowered her record in the semifinals at Energie Verbund Arena, and then battled three Chinese skaters in the final. Meng Wang led from start to finish to take the gold medal in 42.913 seconds, St. Gelais secured second in 42.983, while Qiuhong Liu was the bronze medallist in 43.161.
"I had so many plans in my head (for the final), because I wasn't sure what was going to happen," St. Gelais said. "I had the impression that their strategy would be to block me right from the start line. I anticipated a start that I would have to secure my spot immediately. Finally, it all worked out.
"I managed my race to keep the two other skaters behind me and that Wang wouldn't interfere with me from the front."
St. Gelais skated a Canadian-record 42.664 in the semis, lowering her previous mark of 43.241 set at the Vancouver Olympics.
"I knew it was fast racing today, because you can feel it on your blades and in your legs that you're picking speed in the turns," St. Gelais said. "But you really don't know how fast until you see the time at the end. I was happy when I saw 42.6 seconds, and people have been telling I can go just as fast as Meng Wang."
Meng broke her own world record finishing in the semis, skating 42.597.
"I was following her and I was quite comfortable behind her," St. Gelais said. "I managed that race very well."
St. Gelais teamed up with Marie-Eve Drolet of Laterriere, Que., Valerie Maltais of La Baie, Que., and Jessica Hewitt of Kamloops, B.C., to win the silver. The Netherlands, fourth after 25 of 27 laps, won the gold, while China took the bronze. South Korean crossed first but was penalized for interference.
Drolet and Maltais were fourth and fifth, respectively, in the 1,500-metre final.
Michael Gilday of Yellowknife was fourth in the men's 1,500.