DHA supplementation of 800 mg/d in the second half of the pregnancy does not reduce the risk of GDM or preeclampsia.
So supplementing fish oil does not help. Really, we expected this - there are too many big research hits on immune system issues that are not all that responsive to maternal lifestyle and too many women with spotless lifestyles and preeclampsia to expect to see lifestyle modifications make much difference at all to preeclampsia rates. (Now, to *recovery* rates? That's a different question, and your chances of a good recovery from an acute pregnancy disease are much higher if you've taken good care of yourself.)