According to ESRB, this game contains: Suggestive Themes, Fantasy Violence
I failed to spot the 'Suggestive Themes' that the ESRB did, but there is certainly an abundance of 'Fantasy Violence' packed into Gunstar Super Heroes' short lifespan, as the title itself would suggest. Parents only need be concerned if the concept of gunplay or hand-to hand combat abhors them in any context, because GSH is in no way explicit in its violence and only gratuitous in terms of the resultant explosions and sparks.
Fans of the original Gunstar Heroes should view this sequel as homage, if not a remake, such is the extent of the Treasure recycling plan on display. Newcomers, or those who haven't played the original for years, will have much more fun here, but even they will likely shirk at the game's lack of lasting impact, attributable both to its backwards-looking gameplay and extremely slender game length.
Deaf and Hard of Hearing gamers will miss out on a few bits of in-game character speech, but nothing critical to the progress of either the gameplay or the depressingly awful narrative.