I think one problem is that you have omitted the required attribute
_QUANTITY
in your data.
Your XML data should look like this:
<!DOCTYPE labels SYSTEM "\\pena-ap-008v\GSF_Test_Label\label.dtd">
<labels _FORMAT="GSF_Test_Label\1055_3122_Box_label.lwl" _JOBNAME="f996f9eb-f725-4e59-9e70-fab2edb57ba7" _PRINTERNAME="GSFTEST" _QUANTITY="1">
<label>
<variable name="CustAssNumber">1053-7116<<variable>
<variable name="CustP_N"></variable>
<variable name="CustRev"></variable>
<variable name="BatchNumber"></variable>
<variable name="SerNumber1">JG13080296</variable>
<variable name="DateTime">19 Dec 2014</variable>
</label>
</labels>
If you send the above XML data directly to the printer, a label should be printed.
If this is indeed for a Zebra printer, I don't recognize the extension .lwl in the _FORMAT attribute. I usually use .zpl