Since it is plate you are using, I would use a backing plate with the edges bevelled and a 1/8" gap. Then I would run stringers for the root fill & cap.
Now it is possible to run GMAW with an open root (No backing strip). My company qualified a procedure on carbon pipe. We used .035-70S-hard wire. We set the gap on 3/8" plate at a strong 1/8", used a 37 1/2" degree bevel and hardly any land. Set the machine with 22 Volts and a wire speed of 220 IPM (for starter setting) which we tweaked slightly later and I am not sure what the final settings were. Used 75/25 and travelled Down -Hill. If you play around with the gap (fit-up) and the machine settings you would be surprised at how nice it will look. The problems we had were coming up on a tack , you know making a good "tie-in". It was quite awhile ago since I have done it so maybe someone else can give you more specific information. I hope it works out for you.
1variant
chamfer 2x30degrees,tolerance 1,5-2mm,V-figurative
1layer without padding
solid wire ô1,2 3,4m/min 150A 21V
2-4layers
solid wire ô1,6 6,2m/min 350A 31V gas M21
2variant
1layer without padding
flux cored wire Fluxofil 19 HD/OERLICON/ô1,2
2-3layers
flux cored wire Fluxofil M 10/oerlicon/ô1,2 or 1,6