What happens if Steam revokes your game license?

Yes, you'll get unskippable notification about specific game revoked from your account

One day one of my games was revoked and I get a warning that it was revoked but I couldn't do anything

Getting scared when buying on G2A are ya? /s

Inbefore every cardaddict comes crying about the "2h" rumor CD that ofc doesn't exists.
I've had 5 games revoked I think, 3 betas because devs that know how to make it properly & 2 through SG but it's all solved :)
I did not have a traded gift revoked though, just keys,

You get a notification, you can't miss it, don't worry.

For the second question, if it's a key or a steamgift, I think you won't get a warning or something like that.
But if the problem is caused by one of your cards, IIRC your account could get a temporary block.

http://i.imgur.com/1sh0Tb9.png
you mean those?
I get them very regularly (*not due any fraudulent purchased keys though).

So, don't worry.

I've had a few games revoked before usually betas and a few mishaps with giveaways. However, one time very long ago I traded for a game which got revoked and I did not receive the message on steam. I contacted the trader and they gave me a new key for it...

if already in yr library yes.
if its in yr inventory, no. no notif at all...

If it's in your library, you get a notification. If it's in your inventory, you don't get any notifications.