Susan, if it's the type of steel guttering I'm thinking of, I guess it's quite old.
You would have to cut into the gutter, take a short section out, and replace it with a "running outlet".
Older steel gutter was 4" Imperial measurement. Any good tradesman/handyman could adapt a modern outlet, but you might find it difficult.
Modern fittings are larger. The difference could be made up with extra thickness of seals. If it is old, then I think that a new complete run of plastic would be a sensible investment.