I'm not impressed with the LC+ diesels. What year is your GP-20? I know there were some early ones released that didn't have true bearings- the axles turned in the die-cast truck block. This is shameless cost-cutting. I've heard of issues with the gears on the side of the truck too. After criticism Lionel promised improved designs; I think the very latest production might be ok.
I don't use smoke, so I can't comment on that.