A jock strap is a type of male underwear worn when playing sport. In the film, it is used as a insult. A bit like calling them 'idiots'. It is typical of the slightly rude, derogatory manner of speech common in such situations. The term 'gather up' does mean 'come together'. It is again very typical of a language used by a sports coach. 'Try not to break the camera'. I haven't seen the film, but I would guess it is also a derogatory joke. The suggestion is that the people being photographed are ugly enough to break the glass in the lens!

Personally, I haven't watched 17, but based on the context you provided, I assume that the coach meant to boost the morale of his team... in a casual tone. To gather up basically means to ask people to pull themselves together, and jockstraps is a derogatory metaphor for gays. Also, this sentence can be modified in many ways depending on the context of the scene. In general, I assume it will usually be followed by a peptalk. Hope it helps.