Some writers want to identify 1 John 5:7 as a "scribal gloss," but the spurious text is more than a scribal expansion to me. It has been added to the Bible, plain and simple, as more than an explanatory remark. Describing the religious situation in the nineteenth century, Jaroslav Pelikan wrote: "One group of reasons for the jeopardy into which the orthodox doctrine of the Trinity had come was basically literary and textual. The most pertinacious and conservative in various communions were still holding out for the authenticity of the Johannine comma in 1 John 5:7, despite all the textual and patristic evidence against it, but there was an all but unanimous consensus among textual critics that it represented a later interpolation" (Christian Tradition 5:193).
The New American Heritage Dictionary reports that glosses are usually notes inserted within the margins or between the lines of texts or manuscripts. I would consider 1 John 5:7 more than just a gloss in that sense. It was an addition that totally misconstrued the original intent of the first-century writer, and the passage contributed to deceiving people throughout the centuries in order that they might believe the Trinity doctrine has some merit. The historian Paul Johnson also tried to impute good motives to the scribe who added the Comma Johanneum. See his work A History of Christianity.