You need the sum of the scaled scores so I'd say that, yes, you need the subtest scores. For one dd of mine, her GAI was 7 pts higher than her FSIQ b/c she had one test (PSI) that was much lower than the other three (42nd percentile vs. all other tests in the mid to upper 90s). For my other dd, who had two tests that were much lower (WMI and PSI were avg while PRI and VCI were both at or above the 99th), the difference btwn FSIQ & GAI was 18 pts.
If her WMI was way, way lower than the others or if she's just off by a few points to get into some program that needs a GAI or FSIQ, it might be worth it.