Its not a GEL script that you need its a XOG script.
The "minimum" script* you need to provide is as below ; i.e. you will need to get the right project [color=#ff0000]NAME[color] and [color=#ff0000]ID[color] from your system as well as provide the right [color=#ff0000]custom attribute[color] names.
<?xml version="1.0" encoding="UTF-8"?>
<NikuDataBus xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="../xsd/nikuxog_project.xsd">
<Header action="write" externalSource="NIKU" objectType="project" version="13.0.0.7032"/>
<Projects>
<Project name="[color=#ff0000]YOUR PROJECT NAME HERE[/color]" projectID="[color=#ff0000]YOUR PROJECT ID HERE[/color]">
<CustomInformation>
<ColumnValue name="[color=#ff0000]my_custom_attribute1[/color]">aaaa</ColumnValue>
<ColumnValue name="[color=#ff0000]my_custom_attribute2[/color]">bbbb</ColumnValue>
<ColumnValue name="[color=#ff0000]my_custom_attribute3[/color]">2013-09-13</ColumnValue>
</CustomInformation>
</Project>
</Projects>
</NikuDataBus>
For multiple projects in one XOG file you just repeat all the <Project name... to </project> for each project.
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* - If you are using partitions then the above is not quite enough, you also need the partition info in the CustomInformation section I think. Read out a project using XOG and compare what you get to the above to see what I mean there.
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EDIT :
Its not a GEL script that you need its a XOG script.
What I mean there is that you don't need GEL to just update some data, XOG does this for you.... but if you are trying to build a business-driven process that does this all automagically in the system based on some rules or something like that then yes you might drop into GEL and I have just misunderstood your question :*)