Your source was misinformed.
The DSS for the most part predate Christianity, and are concerned with Hebrew Scriptures (OT) rather than Christian Scriptures (NT). Only one document has been said to possibly be Christian, supposedly a fragment of the Gospel of Mark, which would place Mark as written between 30-60AD. As the only complete word in the fragments is 'and' ... it's a tough argument to make.
There are major copies of Hebrew books, however, as well as documents belonging to the Essenes.
No ... no truth at all.
The Gospel of Thomas is not part of the DSS but of the Nag Hammadi find, and the materials themselves date back to the 2nd century AD.
The argument for the authenticity of the GoT ranges back and forth, but with no hard evidence, it's impossible to say ...
... as the Fathers listed those books which were considered canonical and those which were not, and Thomas is not amongst them, there is a strong argument to suggest that the GoT was never part of the Christian community, nor a serious contender.
It is not fully orthodox, but nor is it fully gnostic, and in fact it's difficult to place it within any philosophical context ... so some put it 30AD, some put it 300AD ... as the only thing going for it is its ambiguity, it has a certain appeal, as one can interpret it any way you like, but it certainly 'misses' the significant 'revelation' of Christian doctrine, so it's not really much use in any meaningful sense beyond the archaeological.
Thomas