Using the recent bladesinger as a model, we can make an essentials-style gunmage that is an easier conversion than making a full class. This might make it an easier sell to a DM.
The bladesinger is a variant of the wizard class and keeps controller as its role, we want a striker, so lets make a sorcerer variant!
Power Source: Arcane/Martial
Key Abilities: Charisma, Dexterity
Armor Proficiencies: Cloth, Leather
Weapon Proficiencies: Simple melee, simple ranged, military ranged
Implements: Daggers, staffs, guns (crossbows)
Bonus to Defense: +2 Will
Hit Points at 1st Level: 12 + Constitution score
Hit Points per Level Gained: 5
Healing Surges per Day: 6 + Constitution modifier
Trained Skills: Arcana. From the class skills list below,
choose three more trained skills at 1st level.
Class Skills: Arcana (Int), Athletics (Str), Bluff (Cha),
Diplomacy (Cha), Dungeoneering (Wis), Endurance
(Con), History (Int), Intimidate (Cha),
Nature (Wis), Perception (Wis)
Let’s borrow the stat block from the sorcerer and add leather and military ranged.
If your game isn’t using guns, you can use crossbows instead, merely reflavor them to taste.
Instinctive Attack > Natural Shooter – use CHA for RBA
Guarded Flourish > Shoot from the hip – don’t provoke OA from ranged attacks
Blade Magic > Gun magic – use guns as implements
Blade Song > Rune Trigger – instead of blade song, Power Strike
Spellbook /Daily Spells > Use Sorcerer Encounter powers as Daily powers
After that we can borrow the bladesinger At-wills with minor modification
Bladespell > Magelock – 3 magelock at-will powers, example follows:
Cold at-will Sorcerer Attack
At-Will Arcane, Magelock, Cold
No Action Ranged implement
Trigger: During your turn you hit an enemy with a ranged basic attack using your gun.
Target: Enemy hit or one creature within 3 squares
Effect: Dexterity modifier Cold damage and the target is slowed until the end of its next turn.
Level 11: 2 + Dex mod cold damage
Level 21: 4 + Dex mod cold damage
Special: You can only use one magelock power per triggering attack
Bladesingers get cantrips and magic missile, neither of which are really sorcerer tricks in this edition, so lets give them something interesting instead.
Cantrips > Alchemy
Magic Missile > Sorcerous Ammo – During a short or extended rest, the Gunmage can create a piece of alchemical ammo up to their class level at no cost. This ammo is consumed when used, or if unused, dissipates during your next rest.
If you want to run with this idea, or develop it further be my guest. This was an interesting deviation, but I prefer my full custom class (bias if a given), which I will be taking further up towards epic after our side quest into psionics.