Is there any chance that source games like Left 4 Dead series, Gary's Mod will be discounted?

Most likely all Valve games will be at least 50% off.

100% sure

Like every valve game is 75% on the big steam sales.....

most of valve games will be 80-90% off i think, like on any other summer/winter sale :D

Yes :D

Yep, they'll both get 75% off a few times in the sale, along with all the other big Valve games.

Yes. I remember getting Left 4 Dead and Left 4 Dead 2 in a bundle on Steam two or so years ago, when it was -75% or -80%.

Left 4 Dead goes on sale on the time so i say 100% chance.