I have added a custom format to /etc/cups/ppd/GK420.ppd in order to match my 70x50mm labels.
*OpenUI *PageSize/Media Size: PickOne
*OrderDependency: 10 AnySetup *PageSize
*DefaultPageSize: w198h141
*PageSize w198h141/70x50mm: "<</PageSize[198 141]/ImagingBBox null>>setpagedevice"
*PageSize w90h18/1.25x0.25": "<</PageSize[90 18]/ImagingBBox null>>setpagedevice"
*CloseUI: *PageSize
*OpenUI *PageRegion/Media Size: PickOne
*OrderDependency: 10 AnySetup *PageRegion
*DefaultPageRegion: w198h141
*PageRegion w198h141/70x50mm: "<</PageSize[198 141]/ImagingBBox null>>setpagedevice"
*PageRegion w90h18/1.25x0.25": "<</PageSize[90 18]/ImagingBBox null>>setpagedevice"
*CloseUI: *PageRegion
*DefaultImageableArea: w198h141
*ImageableArea w198h141/70x50mm: "0 0 198 141"
*ImageableArea w90h18/1.25x0.25": "0 0 90 18"
*DefaultPaperDimension: w198h141
*PaperDimension w198h141/70x50": "198 141"
*PaperDimension w90h18/1.25x0.25": "90 18"
*MaxMediaWidth: "576"
*MaxMediaHeight: "3600"
*HWMargins: 0 0 0 0
*CustomPageSize True: "pop pop pop <</PageSize[5 -2 roll]/ImagingBBox null>>setpagedevice"
*ParamCustomPageSize Width: 1 points 36 576
*ParamCustomPageSize Height: 2 points 36 3600
*ParamCustomPageSize WidthOffset: 3 points 0 0
*ParamCustomPageSize HeightOffset: 4 points 0 0
*ParamCustomPageSize Orientation: 5 int 0 0
Printing from gEdit or LibreOffice, my text is not properly aligned : printing starts well before the left border of the label.
If I print using Windows XP and the Seagull driver, the same .odt file (I use a virtual machine on the Linux host where CUPS is installed) prints properly.
When I return to Linux and print again, the alignment is correct.
My best guess is the the Windows driver sends a config command to the printer, that remains effective when printing from Linux. If I turn off and on the printer, the problem reappears when printing from Linux.
I find it strange that I haven't been able to find any information related to this problem though Zebra printers are quite popular... Am I the only one using Linux to print labels ;-)