Hello,
What you need to be clear here is the Loco Translate plugin and WPML plugin works separately. You can use either of the plugins for the translation.
If you are building the multilingual site then you can use the WPML plugin or else you can simply use the Loco translate plugin for the string translation as it is very handy to use.
Now coming back to your query, the string that you are trying to translate comes from the WP Travel plugin, not the Travel Gem Pro theme. So please try translating the string from the WP Travel plugin. We have attached the screen record for your guidance as well.
https://www.loom.com/share/948b825b47734325925f7f284c7d889a
Also, the travel-related string mostly comes from the WP Travel plugin so try translating the string from there.
Hope this helps.
Thank you.