I can't say I know any specific gels or conditioners that encourage shrinkage, but I do know a couple of techniques that cause my hair to shrink more.

First, applying products to my hair when it is not soaking wet. If I apply products to soaking hair, my curls elongate, but if I apply products to just damp hair, it shrinks up. I believe this is because the products are not sealing as much water into the hair, so the hair is lighter.

Second, plopping. The longer I plop, the more shrinkage I get. I'm assuming this is because the having the hair piled on top of the head means that its weight is not pulling the curls down.

I agree with CurlieCutie that protein can increase shrinkage for some people. This is a great idea for a thread and I hope others chime in with condish/products recs!
Medium/coarse, low porosity, dense, mix of 3s
CG (again) since Nov. 2012

Cleanse: SM Moisture Retention, Elucence MBC, KC Come Clean
Condish: Giovanni DL, Cure Care, Ion Treatment, GVP Balm, AOHR
Stylers: KCCC, SM Souffle, AOMM, SS CEJ, JCRR, JCCC

Trying out steaming!