Start at 5th level.
Why fifth level? Because from long years playing both older D&D and GURPS a 5th level character feels roughly as capable as a 150 pt GURPS Character (100 pts in 3rd edition GURPS). And starting GURPS Character feel about what starting characters feel like in other skill based games like BRP, Hero System, etc.
In D&D, GURPS, etc this represents basically a character who lived a little and now is coming into the prime of his adventuring career.
While I was running GURPS 3rd my rule of thumb conversion rule was 20 pt per level/HD. With 4th edition I use the same formula but add 50pts to the total.
GURPS Gamma World, 20th Homeland - Session 5
1 hour ago