Cabotage basically means using one or more foreign airlines AND not using a domestic airline to travel between two domestic destinations. In the US it is expressly disallowed. GUM is a domestic destination, thus you can't fly CLE-YYZ-ICN-GUM without having at least one of these segments be on a US carrier, as you'd be using two foreign airlines (AC and OS) to go between two US cities. This is similar to how you can't buy a ticket between JFK and SFO via YYZ.

If you can't fit a UA segment on that ticket that you want to buy, you can buy two separate round-trip tickets (e.g. CLE-ICN and ICN-GUM), which is not considered cabotage. Just be aware that you'll have to claim and re-check bags in ICN and you would not be protected in case of misconnects.

See DL's explanation here.