What sort of weight are you envisioning for the final product. I'm assuming this will only protrude a couple of inches or less?
If the weight is not too great a series of suitable sheet metal screws (either through the cross and cover with plugs or secured in keyhole slots). If you need even more support you could even use several molly bolts for greater purchase (just realize you may never be able to get them back out should the need arise)... There are also toggle bolts (if bolted through cross), etc.
If the cross is wide enough and/or especially heavy and the studs are of standard spacing, you might even consider securing it to neighboring studs for greater strength -- many steel studs are not as substantial as their wooden counterparts.
Tip: If you choose to use a series of keyhole slots (either real keyholes or their steel equivalents) it is often much easier to hang if you map them out on a drilling stencil first. Use moderately lightweight paper (such as the *lightweight* kraft paper sometimes used for masking off paint -- usually rolls of 12" or less) and trace the plumb line for your cross, then map out by tracing the shape of each keyhole with black marker. Label this side with the words "WRONG SIDE". Since you used a lighter weight paper your marker should show through the paper. You will want to use this reverse side (the side where "WRONG SIDE" is mirrored) as your drilling template. Secure the template to the wall tight and plumb with masking tape (and "WRONG SIDE" illegible) and center your drill bit in the top of each keyhole outline -- use a center punch if needed. It is also worth using a screw or bolt whose head and shank are one size smaller than the maximum that could fit the keyhole -- this way if you are off-center by 1/32" you will have some leeway.
This saves a lot of headaches trying to measure out the precise locations for each bolt only to discover you were off 1/8" in your measuring!
