As far as we know, the apostle Thomas never wrote a Gospel. The writing to which you refer is spurious and was composed by a member of a gnostic sect, probably in the late second century, which explains why it’s not in the Bible. It was discovered, along with many other writings, in 1946 near the village of Nag Hammadi in Egypt. Our version comes to us in Coptic, but there seems to have been a Greek original behind it.
The so-called Gospel of Thomas really isn’t a Gospel at all, but a collection of 114 sayings or logia attributed to Jesus. Approximately one-fourth of these sayings are the same as those found in the canonical Gospels. Of the remainder, some are very similar to those in our Gospels, but have been adapted for gnostic purposes. Others are completely gnostic in origin and form.
While the Gospel of Thomas purports to have been written by the apostle, there’s nothing to support this, and there’s a great deal that argues against it. To mention only one example (and this example applies to gnostic writings in general), the milieu is all wrong. The peculiar sayings attributed to Jesus in the Gospel of Thomas are, for the most part, light-years away from the kind of thing a Jewish rabbi would have said in the Palestine of A.D. 30. Contrast this with the very Jewish flavor of Jesus’ words found in the canonical Gospels.
For this reason alone, we’re forced to conclude that the Gospel of Thomas doesn’t compare with the canonical Gospels when it comes to telling us what Jesus really taught.