You need to change the 'Start on layout' property of the timeline to make sure it plays in preview. I think our guided tour should include that though, so I'll see about updating that.

Not sure why that error happened when exporting a GIF - I just tried it and it's working here (Chrome/Windows). Could you copy and paste the error message and send it to me at ashley@construct.net? I'll see if I can figure it out from that.