They shout back "Prove that you aren't a shapeshifter or we will be forced to shoot!" Depending on what you do to attempt to prevent being shot, either roll a deception, persuasion, or intimidation check.

Biezdar you can roll a persuasion check based on your message. Quin needs more to be able to convince them.

i think i got it :) rolled 18+2=20 via character sheet

 /i intentionally did not look up the names of gnomes and typed them out of memory, that gave more credibility to my message. You see a spy shapeshifter would not let himself make a mistake like that. Someone honest could :P/

Holding his hands up in front of him in a hushing motion, Quin says, "Even if we could prove it, which is impossible, what would it matter as long as we mean no harm?" 

Persuasion: 13

Jayson calls out.

“Please, sirs, we are in earnest.  We seek to trade for or purchase one of your extraordinary inventions which may help us turn the tables on the marauding dragon that is terrorizing the region.  It was suggested by the townmaster that you might be willing and able to assist us.  The kind lady in your kitchen sent us this way.  May we please approach and parlay?”

((Roll via sheet - Persuasion: 15 + 5 = 20))

They seem unsure at first, but between you all, they are convinced enough to let you pass unharmed. That is, until they see Gor carrying a tied up gnome. "Watch out! You are being followed by a shapeshifter who has Facktoré captured and ready to eat!" And they take aim and fire at Gor. One misses, and one hits, doing 4 points of damage. 
Anyone can try and convince them Gor is not a shapeshifter, not just Gor. 

Quin sees the eyes of the gnomes go wide as they spot their clan mate. Just as he's about to shout an explantation the arrows fly. He moves to position himself between himself and the diminutive guards. Hands up, Quin pleads, "Wait! No! WWe're bringing her to you. She's crazy! Look!" He motions towards Gor and his bundle gnome.

Persuasion: 16

Jayson puts his face in his palm.

”Gentlegnomes, Gor is merely cradling Miss— Facktoré, you said?  He offered her food and inquired as to her gastrointestinal health.  He is not a shapeshifter.  He is in fact a brave, kind, a devoted traveling companion.  We will, of course, return Ms. Facktoré to you.  We were carrying her along because she shot at us and did not seem to understand the King’s common tongue.  But I assure you, we have not harmed her in any way.”

Oh boy, they are all quite quick on shooting things at visitors, it starts to irritate me.

"Hey hey hey, stop with the shooting man or you are going to end up as your wacko friend here!" 

Biezdar is loosing his temper and at this point would rather to quickly end this with some whacking than diplomacy. but still has some reason to believe it might be better to give them a chance to halt their attack.

Intimidation roll=4

They look closer and sigh."Looks like she has got lost in her work again. Sorry about that. She gets really into it to the point of insanity. You just have to let her finish her project and be happy it works. Out of curiosity, what is she working on this time?" 

A thought flies into Kosef's mind. "A crossbow contraption, does she build weapons like this a lot? Professionally?"

"Hey!"  Gor says as he gets grazed by the shot.  "Oah!  What is the meaning of all this?  A shapeshift!  I swear, if I didn't think you people were so darn cute I'd sledge the lot of you into paste!"  

He begins to shake his fist, then realizes he's shaking the tied up Gnome, Facktore, and then stops.  

"Oop, sorry little one...  But who do you think you lot are?  You could hurt someone!  You could put out an ear!  How would you feel if you put out someone's ear, hmm?  Especially an innocent Orc just try to keep this fine lady..."  

He shakes Facktore at them again before catching himself and putting her safely back down to his side...

"...Oah, sorry...just trying to keep this fine lady from hurting herself or someone else?"  

Jayson calls out again.

”Sirs, if we could please return Ms. Facktoré to your care, and possibly adjourn somewhere we can sit and talk this matter over.  The townmaster of Phandalin seems to think your group could be in a position to help us and the town.  Our need is significant.  This dragon is not going to abandon the region of its own accord.  We have to force it out or eliminate it.”

"That is fine. You can let her go. We need to let her go back to finish what she is doing or at least determine that her tinkering is complete. Sometimes a gnome will go for days without eating or drinking when they get lost in work. You may cross the bridge, and talk to Dabbledob and Fiddlesdib. They are the head inventors here." 

Making your way through, you cross the bridge, which is heavily obscured by spray from the waterfall. The room on the other side has two large spinning blades, one going clockwise, one anticlockwise. They look like they will hurt if you try and make your way past. Anyone with a passive perception 15+ notices on the far wall (south wall, past the blades), a lever, currently pointing down.

Map:

/biezdar is not that passively perceptive/ 

"Uh oh, they did mentioned room with blades. I assumed some sort of armory - not this."

"We can try to pass them, but someone might loose his limb in the process. How about we try that other slope on this side? Quinn were you not checking out that slope on the north-west? Were there blade traps as well? "

"No. But the climb is steep. Besides there has to be a way to shut it off. I'll go ask those gnomes."

Quin will wait a few moments for rebuttals or other suggestions before heading off.

Gor looks sad as he begins to untie Facktore and sets her down gently.  

"Aww...I wanted to keep that one."  He said, still trying to offer her something to eat as a peace offering.  "Oah, I hope you have a good day today little one.  I am glad we didn't sledge you."  

When they enter the room with the blades, Gor squints, then pulls out his maul.  

"I can sledge them out of place, maybe."  

Quin hurries back to the two gnome sentries and asks them how to turn off the blades.

"Just pull the lever. It's on the far side. We would go show you but we are not allowed to leave our post until the shapechanger is found" 

 Quin is about to leave when a question strikes him, turning back he asks,  "What is it about shapeshifters that has you so specifically up in arms? Have you been having a problem with one?"