I can't get my gamo whisper mach with a 3×9 sighted in, I've had it about a year and used several different 177 pellets and still can't get it sighted in.

Any commercially available pellet will work. Some styles/weights/manufacturers shoot better in a particular gun than others. Impact points vary with aerodynamics/weight.
I experimented with different pellets to get the most accurate one.
Gamo makes a really good pellet.
Here's a pack with different styles -

GAMO Performance Pellets Combo Pack 177 Cal Multiple Styles 632092854 eBay

PDF]OWNER'S MANUAL - Crosman

www.crosman.com/pdf/manuals/M4-177-515.pdf

Translate this page There are two different rear sight options for the M4-177 depending on which model you purchase.) Option 1- ... the airgun in any way inconsistent with this manual may make your ... Adjust the rings on your scope to match the measurements of the top rail. ... Toattach the sling at both ends push the sewn webbing loop end.

Crosman M4-177 Multi-pump .177 Pellet Air Rifle Table Top ...

? 24:29
https://www.youtube.com/watch?v=s_0grKCOmX0
May 18, 2012 - Uploaded by Replica Airguns
ReplicaAirguns.com - Crosman M4-177 Pellet-BB Multi-pump Air Rifle ... -Single shot - bolt action with manual advance magazine.... I particularly like the removable iron sights and full rail formounting a scope or red dot sight.

Crosman M4-177: Installing and Adjusting the Sights ...

? 8:38
https://www.youtube.com/watch?v=x8XKOmb41QA
Aug 28, 2012 - Uploaded by Crosman
A detailed look at installing and adjusting the sights for the M4-177 and sighting ... Crosman MK-177 Pellet-BB Multi-pump Air Rifle Table Top ...

Crosman M4-177 shooting rev

Bone Collector .177 and .22 cal special edition air rifles ...

www.gamousa.com > Air Rifle
The "BONE COLLECTOR" .177 and .22 caliber air rifles. ... blast reducer integrated into a bull barrel, and utilize the advanced IGT (Inert Gas Technology).
images for Bone Collector .177 Caliber barrel

http://www.airrifleshop.co.uk/Airgun%20manuals.htm
Has some gamo manuals not the specific one you want but has all the important info for safety and operation plus zeroing if its a parts diagram you want look at the following link
http://www.aceros-de-hispania.com/MANUALES-DESPIECES/DESPIECES-CARABINAS-GAMO/despiece-silent-stalker.jpg

Depends on who is shooting. But just test your skills on a 75 yard range. Shooting competition, with the special competition guns are shot at 10 meters (about 30 yards) Without a scoop, they easy shoot 80 % of all pellets through one .177 hole on 30 yards.

I have a similar Gamo air gun and the serial number is on the barrel on a flat spot on the side near the hinge.

The first thing to do is clean the barrel, yes, oil in the barrel will cause accuracy to suffer. Next is get as good a pellet as you can and the flat tipped waisted, kind are the best for accuracy. See picture below --->>>> Next is to lube the pump or mechanism to get as consistent a pump as you can.

no its not normal,,,check that the piston crown is not braking up as if it was the piston would smack into the breach block causing the hard thump on firing