wire greenhouse subpanel
First off -- I would redo the trenching job as it's not deep enough.  18" is the Code specified minimum for electrical and locations vary from 18" minimum cover to 24" minimum cover for gas -- going deeper yet is an option though.  Also: running the gas line inside a conduit sleeve could confuse the next bloke who has to work on this.

As to the co-location of gas and electrical -- it does not appear there are any Code mandated separation requirements for this.  If separation is desired, though, I would use a minimum of 36" horizontal separation (i.e. backhoe bucket width) -- any less, and I might as well have put them in the same trench anyway!

I would put a grounding rod at the subpanel -- Code requires this for outbuildings.  I would then use a 2 pole 30A regular breaker for the feeder breaker and then 15A GFCI breakers in the external subpanel -- using a 2 pole 30A GFCI to protect the feeder sounds OK, until you realize that a long run of wet wire is going to add significant leakage current and could trip the GFCI on its own even with the wire intact due to excess capacitive coupling to the ground wire.

Oh, and I would double check my LB to make sure it's volume marked i.e legal to make the splice in -- it should be, but it's best to be safe than sorry.