I’m not familiar with Met Gala. Is Timmy being a host on the red carpet, or inside the event on the stage? Is the show televised?
No, it isn't televised but the red carpet arrivals are broadcast, usually on YouTube via the Met's page. Being a co-host means he a face of the event for this year, an ambassador that helps to bring attention in order to raise money.
When Harry co-hosted (also on his first time ever attending, I love how their lives continue to mirror one another ❤) this was posted in an article about what his duties would include:
The role of a co-chair also expands into the guestlist, the food, the decor and the general feel of the evening.
In May 2019, Harry Styles will attend his first ever Met Gala - as a co-host. But, what will be be doing? Will be perform? Read Vogue's guid