Yes, it would revert them.
The important wording is already quoted in your question:
All incorporeal creatures in this field become corporeal.
This is an ongoing effect (that is, it applies to any creature entering the area, not just applying once at the time of casting). We know this because the spell's area is listed as an "emanation." From the SRD:
An emanation spell functions like a burst spell, except that the effect continues to radiate from the point of origin for the duration of the spell.
A Psion Uncarnate who is incorporeal (via either the Shed Body or Uncarnate class features) would become corporeal while in the area of a Ghost Trap spell, or upon entering its area. While under its effect, they would lose the benefits described in the incorporeal subtype.
They would not lose their other benefits from the Psion Uncarnate class. For instance, if they were in disguise using the Assume Likeness class feature, Ghost Trap would not force them back into their original form.
The spell text about ethereal creatures has no effect in this case, as an incorporeal Psion Uncarnate is not ethereal.