Thanks to all! Now I think that i will use a rule that gauntlets are not enchanted and if the wearer wants to use them as weapons he must enchant them separately from armor with full enchantment cost.
Interesting thing that i found is advanced armor training option that says
Steel Headbutt (Ex): While wearing medium or heavy armor, a fighter can deliver a headbutt with his helm as part of a full attack action. This headbutt is in addition to his normal attacks, and is made using the fighter’s base attack bonus – 5. A helmet headbutt deals 1d3 points of damage if the fighter is wearing medium armor, or 1d4 points of damage if he is wearing heavy armor (1d2 and 1d3, respectively, for Small creatures), plus an amount of damage equal to 1/2 the fighter’s Strength modifier. Treat this attack as a weapon attack made using the same special material (if any) as the armor. The armor’s enhancement bonus does not modify the headbutt attack, but the helm can be enchanted as a separate weapon.
So i think that gauntlets will count as mithral to overcome DR/silver. But anyway, their wearer isn't proficient with unarmed strike and he will provoke attacks of opportunity while using them.
Maybe it will be incorrect by rules but seems logical.
Anyway, 1d3+STR damage for 7-th level warrior is not a big advantage even if it allows him to bypass DR :)
And it isn't written in rules that armor comes with a set of STEEL gauntlents, it is said "with a set of gauntlets". So i think that mithral fullplate comes with a mithral gauntlets. :)
But maybe they are not "steel" nor "mithral" gauntlets but only "gauntlets". In that case yes, it would not overcome DR i think. Main interest for me was enchantment bonus. It's absolutely clear now that gauntlets are not enchanted as a weapons.
