How to write an effective but gentle reminder email to a journal editor?
Make it a question, not a reminder.

I see no particular reason to worry that your paper has been forgotten. It seems more likely just that yours is scheduled for the May issue (or later in the year) because there were enough earlier-submitted articles to fill up the January issue.

On the other hand, asking politely “Roughly when should I expect my paper to appear?” is a completely reasonable question — there are many reasons you might want to know it — and contains no implied criticism that an editor might take offence at.

And in case the editor has forgotten your paper, this will have the desired effect of a reminder anyway.