Actually the easiest way to do it would be to use it as a Skill Enhancement item (which already exist). Sleight of Hand is a skill that can be used to disguise spellcasting. Gloves of Hidden Casting should be an item that grants a +5, +10, etc. bonus to Slight of Hand skill checks. If you are spotted, then the opposing caster should be able to ID the spell with an equal amount of skill (the gestures havent changed).
Example - Boots of Elvenkind dont use a "Silence" effect, they are a skill modifier item.
Boots of Elvenkind: These soft boots enable the wearer to move quietly in virtually any surroundings, granting a +5 competence bonus on Move Silently checks.
Faint transmutation; CL 5th; Craft Wondrous Item, creator must be an elf; Price 2,500 gp;Weight 1 lb.
Thus...
Gloves of Spell Hiding: These soft silk gloves enable the wearer to hide the somatic gestures made by spellcasting, granting a +5 competence bonus on Sleight of Hand skill checks.
Faint transmutation; CL 5th; Craft Wondrous Item, creator must be an spellcaster; Price 2,500 gp; Weight 1 lb.
Greater Gloves of Spell Hiding: When these gloves are put on, they enable the wearer to easily mask the somatic component of spellcasting. This allows them a +10 competence bonus on Sleight of Hand checks.
Moderate illusion; CL 10th; Craft Wondrous Item, minor image; Price 30,000 gp; Weight 1 lb.
(based off Robe of Blending)
If you wish to restrict the item's use you can make them usable only by spellcasters, or specifically state that they only conceal hand motions and do not aid in more complex acting such as Picking Pockets.
JMHO