How to centre gallery on page

I’m including a shortcode gallery on my page, and am trying to centre it on the page but it’s not working. I’ve used the text centre button, so when I click on the text button, the code looks like this:

<p style="text-align: center;">[su_custom_gallery source="media: 55,53,52,51,50,25,8" title="never"]</p>

Can you tell me what to add to make this centred on the page please?

Thanks v much,

https://wordpress.org/plugins/shortcodes-ultimate/