grand vitara 2dr. 2005 with 115,000 km.
I guess the shock absorbers are gas filled?
when bouncing each corner they feel OK , but when driving, they don't absorb as they used to.
should I check them in a different way?

"Bouncing" is not what they're supposed to do. If you push down hard enough on one corner, it should come back up and NOT bounce at all. If it cycles up and down more than once when you hit a bump or G out, they're probably shot.

The ones on my 92 feel the same way.  Good on a test, but like it bottoms easily when driving.  In fact every Tracker I've ever driven has felt this way new or old.  I'll probably replace the rears since they aren't supposed to be that hard to change.

there all easy to replace if the bolts are not froze

if you want a sturdy setup pick up the Bilsteins.  A bit more expensive then the standard auto store replacements but worth it.  I put some Monroe struts on the front, one trip to Oregon and they felt like the stock units after 12 years.  The Bilsteins held up to being raced last year at the Zukiworld Challenge, so I'm not surprised they have held up in my DD so far without any problems.

115,000 Km is considered reasonable enough for shock replacement?
anyone had KYB shocks? are they  considered OEM?

I got KYB Mustang rear shocks, and I ran KYB stock shocks before the lift.  They're fine for the price,  ride good enough for me, and reasonable durable.  I think KYB is still considered OEM.