[Rupert]

Note that a 1st level D&D 3.5 character with a Stat of 10-11 and max ranks (4) in a skill for a total of +4 has a 50% chance of making a DC15 check, and DC15 is what d20 systems tend to think of as a average check. Thus +0 for DC15 is about right - someone with some training an no great talent will have about skill-10 in GURPS.

D&D 3.5 tends to consider an situation that would give a modifier worth a +-2, so where something would give that, give a +-1 in GURPS (masterwork non-combat gear does this,m for example, while 'good' equipment gives a +1 in GURPS).

Jumps in DC in D&D usually come in 5-point chunks unless part of some formula, while B343-344 uses 2-point increments for difficulty, so each +5DC would give a -2 penalty (as other posts also suggest). If this doesn't seem to make life hard enough, just go to -3 per step.