Hi,In the web_editor module, there's an xml file editor.xml inside views. Inside that there's a div with data-name="predefined_gradients", the gradients are set up from here.
You can inherit this template and add your gradients using xpath.
<template id='custom' name='Custom' inherit_id='web_editor.colorpicker'>
<xpath expr="//div[@data-name='predefined_gradients']" position="replace">
<div class="o_colorpicker_section" data-name="predefined_gradients">
<t t-set="gradients" t-value="[
'linear-gradient(135deg, rgb(255, 204, 51) 0%, rgb(226, 51, 255) 100%)',
'linear-gradient(135deg, rgb(102, 153, 255) 0%, rgb(255, 51, 102) 100%)',
'linear-gradient(135deg, rgb(47, 128, 237) 0%, rgb(178, 255, 218) 100%)',
'linear-gradient(135deg, rgb(203, 94, 238) 0%, rgb(75, 225, 236) 100%)',
'linear-gradient(135deg, rgb(214, 255, 127) 0%, rgb(0, 179, 204) 100%)',
'linear-gradient(135deg, rgb(255, 222, 69) 0%, rgb(69, 33, 0) 100%)',
'linear-gradient(135deg, rgb(222, 222, 222) 0%, rgb(69, 69, 69) 100%)',
'linear-gradient(135deg, rgb(255, 222, 202) 0%, rgb(202, 115, 69) 100%)'
//Your custom gradients in list form
]"/>
<t t-foreach="gradients" t-as="gradient">
<button class="w-50" t-attf-style="background-image: #{gradient};" t-att-data-color="gradient"/>
</t>
</div>
</xpath>
</template>
Hope it helps