You cannot modify built-in galleries. You should create a Custom gallery e.g.
Code:
customUI xmlns="http://schemas.microsoft.com/office/2006/01/customui">
<ribbon>
<tabs>
<tab id="Tab" insertBeforeMso="TabHome" label="Custom1">
<group id="Group2" label="LL" >
<gallery id="CustomAutoTextGallery" label="Lunge" size="large" image="Lunge" />
</group>
</tab>
</tabs>
</ribbon>
</customUI>