Ok here is what i would do..
Loop at gt_header into gwa_header.
Clear gwa_final.
pass gwa_header into gwa_final.
Loop at gt_item into gwa_header where xxxx = gwa_header-xxxx.
Pass gwa_item into gwa_final.
Append gwa_final to gt_final.
Clear gwa_final.
Endloop.
Clear gwa_final.
Endloop.
The key here is you are clearing gwa_final inside the item data.
So the header data will be passed to your final table only for the first time and on subsequent loops only item data will be displayed in your final.
Once the header data gets looped again you will get the first record with header details..
And use this table to display ALV.
Hope you got my point.