Looks like a nice solidly built gate,good material thickness, you should not have a problem welding it with 6013, Just be careful wear a mask when grinding/welding because of the galv,
Grind the area to be welded to remove the coating of galv. and make sure you wear a good respirator while grinding and welding. I would use either 6010, 6011 for the root and then fill and cap with 7018. 6013 is not a deep penatration rod. I would also bevel the area to be welded and weld full penetration. Of course this is just my .02 cents
When you get zinc hot, it gives off fumes that are extremely nasty. You can get flu like symptoms, fever, headaches etc.
So it's best to remove it before welding by grinding it off in a well ventilated area.