Per GraphQL wiki I can see that there is the only DateTime type: Link
As far as I understood to make only time there should be written additional scalar for that, but I do not have any clue how maybe somebody had already done that and could share his/her ideas? :)
As currently if I will use ISO8601DateTime I don't think it will work out as I'm using time column in database same as in ruby
You can look into this blog post on how to create a custom scalar in GraphQL Ruby and it actually shows exactly how to do a time scalar.