[sir_pudding]

The easiest way is probably just compare the Task Difficultly Modifiers to the generic DC table and match similar descriptors. So:
DC 0=Very Easy=+6 or +7
DC 5=Easy=+4 or +5
DC 10=Average=+0
DC 15=Tough=Unfavorable=-1
DC 20= Challenging= Hard=-4 or - 5
DC 25 =Formidable= Very Hard =-6 or - 7
DC 30= Heroic= Dangerous = -8 or -9
DC 40 = Nearly Impossible = Impossible = -10

The difficulty with a more rigorous approach might be to say that to have a 50 % of DC 10 you need skill 0 which would therefore be statistically equivalent to Skill-10, but that means that untrained skills in D&D are better than GURPS defaults by about 40-48%. If instead you assume that default use is equivalent, "Average" difficulty in D&D is actually equivalent to Very Easy to Easy in GURPS, which makes it non-trivial to assess what the equivalent of Skill-10 is. At any rate once you have decided on what Skill-10 is equivalent to you can use that baseline and the fact that each +1 to DC is - 5% chance of success and compare that the Probability of Success table and then subtract 10 to get a modifier.

Assuming +0=Skill-10 then:
DC 5=75%=~Skill-12=+2
DC 10=50%=Skill-10=+10
DC 15=25%=~Skill-8 = -2
DC 20=5%=~Skill-5 = -5
And then you run into the problem that nothing higher is even possible to roll on 1d20 whereas 3d6 goes down to half a percent.