I need to override
/vendor/magento/module-catalog/view/frontend/templates/product/view/gallery.phtml.
For this, in app/code/namespace/Customtab/view/frontend/layout/catalog_product_view.xml
<?xml version="1.0"?>
<page layout="1column" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:framework:View/Layout/etc/page_configuration.xsd">
<body>
<referenceContainer name="content">
<referenceBlock name="product.info.addtocart">
<action method="setTemplate">
<argument name="template" xsi:type="string">namespace_Customtab::product/view/gallery.phtml</argument>
</action>
</referenceBlock>
</referenceContainer>
</body>
paste the custom phtml file in
app/code/namespace/Customtab/view/frontend/templates/product/view/gallery.phtml.
But it didn't works.Please help me to find a solution.