There are two standard editions of the Greek text of the New Testament. These are the Nestle-Aland edition (aka Novum Testamentum Graece) and the United Bible Society edition. These are in their 28th and 5th editions, respectively. The Greek text is identical in the two versions, but the apparatus is different. NA 28 contains more notes regarding textual variants, UBS 5 contains fewer notes, but they are more substantial.
Here is a "User's Guide" to the Nestle-Aland Greek New Testament. This resource helpfully explains how to use the apparatus and get the most out of the critical edition.
Gospel Parallels are books that present the text of the Gospels side-by-side to aid comparison among the evangelists' accounts of similar (or unique) stories in the Gospels. These resources mostly involve the Synoptic gospels, but some include relevant material from the Gospel of John.
Parallels offer the reader the ability to quickly see at a glance how particular pericopes differ between the gospels. By utilizing the insights of source criticism, inferences can be drawn that highlight the particular changes or edits made to the Gospel tradition by individual evangelists, and therefore aid an understanding of authorial intent.