Are thee any accessible apps that automatically convert from your time zone to UTC/GMT time? I've looked for something like this on the built-in clock app, but am unable to find a way to do it.
I'm not sure if this is what you're after, but you can just go to google and type into the search field time:name of the city you wish to know the time to.
I just tested it to find out if it works for GMT and it does.
So for example you want to find out the current time in GMT you would simply type time:gmt.
I don't know if this will work on an iOS device, there's no reason why it shouldn't.
There is no need for a separate app for this. Using the built-in clock, you go to "add". Either there is a list of choices, or you can type UTC, I don't recall which, and that time will be added to your favorites.