Efforts to debunk the crucifixion (it was done on a stake, on a tree, the cross has pagan origins, etc.) stem largely from groups that seem bent on undermining traditional Christian teaching (most famously, Jehovah's Witnesses), or from scholars who are suspicious of Christianity, in general.
Yet, Roman crucifixion is historical, and it was gruesome.
The Crucifixion of Christ: Roman Practice
If we find out that ultimately it was not a cross (it was a tree or a stake), then it really won't effect the gospel message. However, I'm pretty confident it was...and the truth of it makes the Passion of the gospels all the more poignant, imho. It's not negativity, it is the
gravitas of Jesus' love for us and the Father's deep hatred of sin and rebellion.