What kind of bouncing? If you mean really jiggly then that sounds like a daft and pointless game mechanic. If you mean smooth bouncing, a'la a more subtle, stationary portal 2 launch pad, a tall func_push with a large upwards force should be sufficient. I havn't used func_push since HL1 though so they might have changed its dynamics so that it wont override gravity. You'll have to test that yourself.
It propels anything you want it to propel.Does the trigger push at least propel players?
So, I tried creating it again, tries with trigger_multiple's, but I can't get it to make it so no matter the velocity you are falling at, once you touch the bounce plat, you'll get the same height you fell at, like repulsion gel (or was it propulsion?)
Thanks for helping me, though.
I wonder if it's impossible to recreate the gel into TF2.