No. of Recommendations: 4
A tricky question, and one I'm not equipped to answer thoroughly. I did a bit of reading, and it appears that while gravity bends space/time, magnetic fields attempt to flatten it out. However, a magnetar is a neutron star, which is very dense, and provides a significant warp of space/time due to its mass. Those would be competing effects.
This was the shortest, simplest explanation I could find. It is really fascinating, and I didn't know about this.
science.org - Magnetism stiffens space time 