On Evo, you can only have the donkey bike if you have the "Unyielded" achievement from Trials HD.
Now, how about Gecko, but it's not possible that Evo is required to unlock, because of PS4, when there wasn't Evo on it.
I think maybe that you need to unlock a special achievement, that could be very hard, like gaining a dozen of Platinum medals. Could be also something similar to unlock the Donkey.
Lets discuss what it could be to unlock the extremely wanted Gecko bike...
They could just do a progress achievement, or just not do this at all and make it come standard. I love the gecko, but I'd much prefer getting a B license to acquire it to doing something like gaining a specific amount of platinums or 0 faulting a hard tournament