Gurps roll-over
Base rule: 3d6+skill <=21
Critical succes
A roll of 17 or 18 is always a critical success
A roll of 16 is a critical success if your effective skill is 15+
A roll of 15 is a critical success if your effective skill is 16+
Critical failure
A roll of 3 is always a critical failure
A roll of 4 is a critical failure if effective skill is 15 or less
A roll of 11 or less (including effective skill) is a critical failure
If skill is 5, a roll of 6 or lower is considered a critical failure
Health tracking:
Count wounds from 0. Each multiple of HT gives -1 to the check etc.
Other thoughts
Each attack consists of one attack roll from the attacker and one defense roll from the defender. With a pure roll-over conversion, nothing will change apart from the roll-over rule. That would also mean that the possibility of a long combat will increase since a high defense makes it possible to counter every blow.
One roll option would be to make the attack roll contested (3D6+Attack vs. 3D6+Defense). I don’t know how that would affect the statistics though.