DS skipped 6th grade math and did pre-algebra this school year. Because I wasn't certain whether he should be accelerated again, I actually had him run through ALEKS pre-algebra last summer. It took him about a month to complete it with very occassional input from me and it gave me the confidence to request the acceleration to pre-algebra at school.
I think ALEKS is a good starting point for you to verify that she should skip 6th grade math. You can have her take the assessment for free to see if she has enough of the pie. The ALEKS pre-algebra was reasonably broad and it provided an analysis based on each state's standards. However, I caution that you want more for a gifted student. Even though DS completed the ALEKS pre-algebra last summer, he actually gained a deeper and broader knowledge/understanding from doing pre-algebra again in school because that program is geared toward gifted students.
I haven't tried it, but the AOPS program looks more robust.
