The spell is pretty straightforward even from the name, but let's see the description of the spell from GURPS.
The subject becomes unable to lie, though he may keep silent or tell partial truths (this must be roleplayed). The spell does not force him to volunteer information; he merely cannot say anything he believes to be untrue.Making someone unable to lie. This is almost the Truthfulness disadvantage from the basic set. The ability is an affliction that seems more realistically resisted by Will than HT. Pure Truthfulness is a self-control disadvantage, so we need to modify it so that the roll automatically fails, which will make this spell more expensive. On the flip-side, Truthfulness doesn't even allow lying by omission, which Compel Truth does, so we can add a limitation to disable that and make it cheaper. The final spell is as follows:
Compel Truth (+91%): Affliction 1 (Will +20%; Disadvantage, Modified Truthfulness +11%; Increased 1/2D, 10x, +15%; No Signature, +20%; Sorcery,‑15%; Extended Duration 10x +40%; Fixed Duration 10 Minutes +0%) 
Note, modified truthfulness is Truthfulness (Irresistible +150%; May Remain Silent -20%; Magical -10%) 
When cast, a user resists with Will, if they fail, they may not lie for 10 minutes. The only defense available is simply not to speak.