The joke is that Kimina shouts 'UNION!' like what shows up so often in mecha anime (i.e. the robots joining together at some point) and then the screen splits to show the reactions of every character present on the battlefield: "Union?" "Union?" "Union?" "Union!" "No way!".
This, of course, is followed by Kimina slamming Lagann on top of Gurren in a "Poor Man's Fusion" and a massive sweatdrop moment.
Gattai can also mean to "hook up" with someone, which why Yoko was blushing (this is a reference to a similar scene from Magical Shopping Arcade Abenobashi). That's why she isn't very happy at the result.
JinSaotome said: Gattai can also mean to "hook up" with someone, which why Yoko was blushing (this is a reference to a similar scene from Magical Shopping Arcade Abenobashi). That's why she isn't very happy at the result.
So, uh, does this mean Yoko is a bit of a yaoi fangirl or something?