Add TravelgateX to list of GraphQL users

[arquio]

TravelgateX is a collection of GraphQL APIs for the travel trade.

Thanks!!!

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[arquio]

#644

[caniszczyk]

We will be moving to a new landscape+page to track GraphQL users:
https://landscape.graphql.org/category=graph-ql-adopter&format=card-mode&grouping=category

Can you please add your entry here via pull request:
https://github.com/graphql/graphql-landscape#new-entries

You can see this example with Stackshare as an example:
graphql/graphql-landscape#48