Red Gels are used to unlock further sections of upgrade paths while in the wheelchair in Sebastian's Room. Here's my guide for all of the collectibles in the game. If you want the time stamps for all collectibles, you'll need to open the video in YouTube (click the title).
Chapter 3:Red Gel 1 - 13:00
Red Gel 2 - 16:22
Chapter 5:Red Gel 3 - 26:14
Chapter 6:Red Gel 4 - 31:34
Chapter 8:Red Gel 5 - 40:31
Chapter 11:Red Gel 6 - 47:27
Chapter 13:Red Gel 7 - 52:53
Chapter 14:Red Gel 8 - 1:01:14
Chapter 16:Red Gel 9 - 1:03:41
In a Locker:Red Gel 10 - 1:05:26