@team blueplant  
 
The key word here is RELATIVE. The speed its travelling is irrelevant since the pony is travelling at the same speed right next to it in the same direction. just like how both the gun and pony are travelling through the universe at countless kilometres per second, but that’s never an issue, is it?
 
Imagine telekinesis like an extending limb from the unicorn’s horn, or from whatever frame of reference is more steady for whatever purpose they need.  
You’re thinking of it as if the frame of reference was the floor/room they are in. Imagine if they were to use telekinesis in a moving vehicle, they’d set the frame of reference to, again, themselves, or the vehicle.  
Imagine if they set the frame of reference to the center of the universe, the moment they grab it, it’d fling off in whichever direction opposite to the direction the planet is travelling relative to that frame of reference like a bullet. This of course assumes the connecting force of this arm is rigid, and not flexible, otherwise it’d be a more gradual change in momentum regardless of the frame of reference.
 
alternatively, telekinesis could literally just be applying forces however they want, in which case gradual changes in momentum can be applies as they wish, so again, moving at the same speed and direction as the gun makes this easy.  
honestly they could just stop both the gun and themselves from falling if they thought about it. unicorns have no issue picking up other ponies with telekinesis, nor themselves, as shown by moondancer or starlight.

@zippysqrl  
but i mean, since the portal gun is probably falling at a high speed, grabbing it with magic would stop it in place? or it will gradualy loose speed until its standing still in place?

@team blueplant  
Didn’t look at the image long enough to realise they were a unicorn, and what you said implied they wouldn’t be able to use magic anyway.  
But since their positions are relative and moving at the same speed, there’s no “breaking of physics”, so that isn’t even an issue and just grabbing it with telekinesis would be fine.

@zippysqrl  
or, you know, use too-op-pls-nerf unicorn magic and grab the gun, but lets not break the physics :P

Air resistance slows descent, increase or decrease drag by changing the angle of your body to catch up with or slow down to meet the gun, easy.

You dun goofed.

now yerr stuck forever in an endless fall
 
unless the portal gun falls in your head or chest area