Is it possible to have signature only in the 1st message in Gmail? Outlook has this feature. This is annoying if you have a large signature in gmail.
No, sorry, but Gmail doesn't offer a way to control whether your signature is added in a new message differently than in a reply. You'll just need to remove the signature manually if you don't want it in subsequent messages.
Many people use the "canned response" lab to create snippets that they use for signatures, but you need to remember to add it. (It's just a couple of clicks, but still...)
I can think of a way to fake your way to doing this, but it requires having multiple "from" accounts for the same email address (since you can define different signatures for each of your sending accounts) but that still would require remembering to use one address for the initial message and another for replies.
The other obvious solution is to use Outlook as a client for your Gmail account.