I used an RTA module exactly like that to communicate between a CompactLogix L16ER and a DataMax label printer. I set everything up in Logix5000 according to RTA's documentation. I was able to get data to pass to and from my PLC to the gateway using the RTA web page, but just could not get it to go to the printer; or so I thought. After many different serial cables, I finally discovered that my problem was that I was sending the ASCII characters to the printer in the wrong format. Initially I was trying to send it data like "%02n$r$1, %02M0500$r$1, etc." but what I actually needed was "[1]n, [1]M0500, etc."
Long story short, check all of the manuals you can get your hands on, and make sure the ASCII syntax is correct.