Hey does 5 Groomstalker/15 swashbuckler work?A friend of mine is running a campaign and not allowing the flanking rule that give advantages this time.I still want sneak attack and Swash gives it as long as it only 1 enemy within 5 feet.

will this multi class work?

Regular sneak attack: "You don’t need advantage on the attack roll if another enemy of the target is within 5 feet of it, that enemy isn't incapacitated, and you don’t have disadvantage on the attack roll."

Swashbuckler: "You don’t need advantage on the attack roll to use your Sneak Attack against a creature if you are within 5 feet of it, no other creatures are within 5 feet of you, and you don’t have disadvantage on the attack roll. All the other rules for Sneak Attack still apply to you."

So you would be able to get sneak attack with flanking regardless. Ranger doesn't really factor into this.

This campaign we don’t have flanking so how would I proc sneak attack?

Advantage isn't the only way to get sneak attack.

"You don’t need advantage on the attack roll if another enemy of the target is within 5 feet of it, that enemy isn't incapacitated, and you don’t have disadvantage on the attack roll."

You just need an ally to be within 5 feet of the enemy. If you don't have an ally, and it's just you and the enemy and no one else within 5 feet, you can sneak attack, as a swashbuckler. So if you have an ally flanking the enemy, they're within 5 feet of the enemy, giving you sneak attack, but not within 5 feet of you, so the swashbuckler rule doesn't kick in.

Yes. I built a Human Swashbuckler 4/ Twilight Cleric1/ Gloomstalker 5/Swashbuckler10. I build a templet when I have an idea for a character.

I love playing my Dual-Classed Gloom Stalker / Swashbuckler 8/6 (will keep raising this until 15+)
I didn't follow anyone's advice on this. I don't care if crossbow expert adds slightly more damage per second, etc. I am having a lot of fun with this character though.

Wood Elf
Gloom Stalker
@2nd - Two weapon fighting style
@3rd - Umbral Sight (90' Darkvision) & potentially invisible to those who rely on Darkvision (requires full darkness)
@3rd - Dreaded Ambusher +10' movement & +1 attack on 1st round of combat only (this extra attack specifically does 1D8)
@4th - Dual-wielder
@5th - Extra Attack
@7th - Iron Mind (add proficiency bonus to WIS saves)
@8th - Lands Stride (move through difficult terrain, etc.)
@8th - ASI (+2 DEX)

Swashbuckler
@1st - Expertise in Stealth & Tools
@1st - Sneak Attack 
@2nd - Cunning Action (Dash, Disengage & Hide)
@3rd - Fancy Footwork (like a disengage w/o spending an action)
@3rd - Rakish Audacity (no longer need Advantage to apply Sneak Attacks)
@4th - Sharpshooter
@5th - Uncanny Dodge (use this every round)
@6th - Expertise in Slight O Hand & Acrobatics
@7th - Will get Evasion 
@8th - Plan to take Piercer (+1 DEX)

NOTE: If you use Tasha's Guide @1st - You gain Canny (like Expertise) and then can have double your proficiency bonus on Perception. You can also increase your walking speed by 5 @6th level ranger.

Clearly not the most OP. I didn't plan this build. My toon evolved into this as the game progressed. Originally, I created him to be an Underdark hunter, specializing in tracking down the Drow. LOL. Yeah, a racist elf - playing to the true description of hating evil elves. Later, I even got into the role-playing a bit. As we killed several of the Red Wizards of Thay I had one of their red robes tailored into a musketeer tabard. Soon afterwards, I started stitching skull & crossbones for each Red Wizard we killed. This resulted in an impressive scorecard on my toon's chest. He now has a little bonus CHA towards those who dislike the Red Wizards and has a negative CHA towards Thay sympathizers. LOL.