Try to leave the abandoned town and enter again. If you go to the forest park and Gin is there, leave the town and enter again. If he is gone from his usual spot DON'T go to the lake. I noticed that no matter how many times I enter and leave the town, Gin stays gone if you don't go in the lake. Though I'm not entirely sure if that's the case or if I'm just lucky.
Anyway, if Gin is gone, don't go in the lake, go to the safehouse to see if Cassie is gone too. If she is there, leave the town and enter again without meeting Gin. If she's gone, then you have a huge chance that they're both in the lake. If not, repeat.