Some of the premade outfits have gloves: Michael's jewelry store heist suit, and a few of Franklin's Uptown Riders motorcycle outfits. Altering these outfit will remove the gloves, so unfortunately you can't have gloves on your custom outfit. Some of the premade outfits have things that can't be worn normally, like Franklin has a few pairs of pants that don't appear in his wardrobe and only come on the premade outfits. But sometimes you can get things from the outfits. For example, many of Franklin's "ghetto" outfits have him wearing bracelets and a watch. There is no menu option to equip these, but if you choose an outfit that has these and then alter it to equip your own choice of clothes, he'll still have the accessories. The same is true with Trevor and the digital watch that is on some of his outfits.