McGonagall doesn't create the sandwiches with magic, and neither does the plate. She essentially summons/displaces a plate from the Kitchens or Great Hall, which is magically linked with identical plates in the Kitchens. The House Elves make the sandwiches, place them on the plate in the Kitchens, and then they're magically transported to the paired plate elsewhere (usually on the tables in the Great Hall).
There's a description of the kitchens in Goblet of Fire, Chapter 21 - The House-Elf Liberation Front:
he [Dobby] seized Harry’s hand and pulled him off into the kitchen between the four long wooden tables that stood there. Each of these tables, Harry noticed as he passed them, was positioned exactly beneath the four House tables above, in the Great Hall. At the moment, they were clear of food, dinner having finished, but he supposed that an hour ago they had been laden with dishes that were then sent up through the ceiling to their counterparts above.
As far as Mrs. Weasley's wand producing sauce, I really don't know how that works. That happens in Goblet of Fire, and Gamp's Law of Elemental Transfiguration doesn't appear to be mentioned until Deathly Hallows (three books later). Here's what Hermione says about it:
“Your mother can’t produce food out of thin air,” said Hermione. “no one can. Food is the first of the five Principal Exceptions to Gamp’s Law of Elemental Transfigura—”
“Oh, speak English, can’t you?” Ron said, prising a fish out from between his teeth.
“It’s impossible to make good food out of nothing! You can Summon it if you know where it is, you can transform it, you can increase the quantity if you’ve already got some—”
It could just be a case of JKR being inconsistent, and not having thought up Gamp's Law of Elemental Transfiguration (and its exceptions) prior to that point. Not being able to magic up food was important to the plot (it was one of the things that created tension between the trio) during Deathly Hallows, but hadn't been at any point before that since they'd always been at Hogwarts eating (usually) three meals a day.
