Just look at its popularity now. NOTHING in comparison to Geass, and a lot of Lucky Star/Geass fans are BASHING Gurren-Lagann. A lot of "mainstream" fans don't like the look or think "LOL GAINAX" and never give it a chance.
Gurren-Lagann resonates GAR so it's an /a/ favourite, but that's not the case in many other communities. Hell, the only other place I've seen GL comparable popular to /a/ is, of all places, NarutoFan.
ADV will promote it a lot but ultimately GL will remain an obscure title.
Which is wonderful for /a/, since I wouldn't be able to stand myself if stuff like Yotsuba, Akagi, or GL were merchandised to hell.