No that's really dumb. She best be credited as Lady Gaga. They're using Lady Gaga in the teasers..

She was credited as Lady Gaga in Sin City, and Machete Kills if she isn't credited as Lady Gaga, then what are they actually thinking (Gaga included) people who sit down and watch the opening credits who have no outside knowledge of Gaga will know that Gaga is actually Stefani, and if so, it'll look like she isn't credited at all, which is pathetic.
I honestly don't think she'll be credited as Stefani, I'm sure it's just for her scripts, just like her passport would say Stefani Germanotta, they probably didn't know how to address her.