The Grand Alliance standard differs from all existing TV standards in three major ways. First, it is all digital standard - to be broadcast with a packet transmission. Second, it supports multiple formats. Third, it is designed to be primarily compatible with computers rather than existing NTSC televisions.
Summarizing the various formats:
active lines active aspect ratio frame rate in
horizontal Hz*
pixels
720 1280 16/9 progressive 24, 30 or 60
1080 1920 16/9 interlaced 60
1080 1920 16/9 progressive 24, 30
* Spectrum reports that "all the formats are supported with NTSC frame rates, 59.94Hz, 23.97 Hz, and 29.97 Hz."