When connecting trådfri to openhab, do I need the trådfri hub/gateway or is a bulb enough?

You need the gateway

Also see this post: IKEA Trådfri Gateway
After I changed my patch-cable all has been OK.

Earlier in the same thread: IKEA Trådfri Gateway
No need for a remote if you have a Gateway. (but it is always handy as a backup if/when OH2 is down :slight_smile: )