social/base_search_mail_content/data/trgm_index_data.xml

<?xml version="1.0" encoding="utf-8" ?>
<odoo noupdate="1">

        <record id="subject_gin_idx" model="trgm.index">
            <field name="index_type">gin</field>
            <field
            name="field_id"
            search="[('model','=','mail.message'),('name','=','subject')]"
        />
        </record>

        <record id="body_gin_idx" model="trgm.index">
            <field name="index_type">gin</field>
            <field
            name="field_id"
            search="[('model','=','mail.message'),('name','=','body')]"
        />
        </record>

        <record id="record_name_gin_idx" model="trgm.index">
            <field name="index_type">gin</field>
            <field
            name="field_id"
            search="[('model','=','mail.message'),('name','=','record_name')]"
        />
        </record>

        <record id="email_from_gin_idx" model="trgm.index">
            <field name="index_type">gin</field>
            <field
            name="field_id"
            search="[('model','=','mail.message'),('name','=','email_from')]"
        />
        </record>

        <record id="reply_to_gin_idx" model="trgm.index">
            <field name="index_type">gin</field>
            <field
            name="field_id"
            search="[('model','=','mail.message'),('name','=','reply_to')]"
        />
        </record>

</odoo>