Yes, and no.
It's realistic because it makes perfect sense. Pau Gasol is worthless in the D'Assholio system. As Knicks fans we've seen how many players that were unused or poorly used here but went to other places and became succesful players. (Corey Brewer, Jordan Hill, Darko, Brian Cardinal, Nate Robinson, ect).
Pau Gasol's case will be very similar to Carmelo's. Both of them thrive in a half court system and get a lot of their point on post ups. In Pau's case if you take away post ups and working on the boards then all he really has is a mid ranged jumper.
With Amare, the Lakers would have a second pick and roll option with Dwight, and It gives Nash another weapon on the fast break, which the D'Antoni system is all about.
The D'Antoni system's answer to defense is more offense, and in a system where it's all about speed led by Steve Nash and Mike D'Antoni, you're not gonna have problems with Dwight and Amare clogging up the paint as an earlier poster suggested. Just more fast paced offense. and unlike the Knicks offense the Lakers have three other guys to go through when they need scoring. Amare won't have to do as much as he does here where he is the second scoring option. Which will keep him and his old man knees much healthier.
Pau Gasol fits into Mike Woodson's Knicks aswell. He's a great defensive player who can rebound, post up, and shoot a mid range J. He fits with Melo who also thrives in a half court offense instead of the speedball offense Amar'e thrives in.
Pau doesn't need someone to create for him as well. He can play off the ball unlike Amare. Melo can continue to be a superstar with Pau and score 30 a game while Pau will score a quiet yet efficent 15-20 pts, and will grab all the rebounds he wants. This team desperately needs someone to pull down boards and Pau can fill in on that task as well
So it's a perfectly realistic trade. It makes sense for both teams and players and If i was ether GM I would be trying to get this done as soon as humanly possible. But the thing is there are plenty of logical NBA trades that would benefit both teams that just haven't and may not ever happen. Hopefully in this case it gets done. I give it a 50/50 shot of happening within the next two seasons. 25% chance it happens this year.