Can I play an animation on the player current weapon by script? Not a "default" animation like reload, fire, etc.

You can make a new weapon and change the animations used for that weapon.

But if you mean play an animation on the current viewarms holding the current weapon, the easiest way I achieved that was through additional weapon files with different fire and reload anims. I would forceviewmodelanimation on the other weapon file. Settings get tricky.

But an easier way to do that with a weapon file is to just make the weapon file an alt of the weapon, and in the alt animation you put the animation you want to play. You would script it to switch back, and this wouldn't work with weapons that had alt weapons already very easily. Plus you have some menu editing for the alt image and counter to not display.

I don't know what that is, but would maybe earthquake work for that? It shakes your whole screen... Or is it like an actual shaking of the gun or something?

If you use the alt weapon thing, just freeze switching, switch to alt weapon, wait for it to be done anim, and switch back, and un freeze, I would think. You will have to add checks for it though, maybe with a waittill_any for weapon changes and complete weapon changes in a loop to make sure it doesn't get stuck, and doesn't get given back when they are downed or something.

Just a quick shake of the gun, thanks for the tip

Could maybe use just the dvars cg_gun_x/y alternatively very quickly to simulate the viewmodel moving from side to side?

How could I use that?

function name()
{
while(1)
{
currentweapon = self GetCurrentWeapon(); 
if( ( currentweapon == "portalgun" || currentweapon == "portalgun_upgraded" ) )
{ 
self setClientDvar( "cg_gun_z", -1.5 );
}
else
self setClientDvar( "cg_gun_z", 0 );
wait .1;
}
}

I've heard that is a host only dvar, is that true?

Really thanks, saved me from doing another animation on maya 

Idk how it is supposed to look (never played any of the Portal games) but honestly I think MakeCents suggestion about using 'earthquake' is the simplest and best solution.