Hammerli made 11 different Pellet gun models. Unfortunately you did not state the model number so here is the best I can do with out any information.
See the link below
No. a 177 is a 4.5mm pellet. A 5mm is a 20 cal pellet. A 5.5mm is a 22 cal pellet Don't try it in the wrong airgun.
As long as the pellet is .177 Cal then any .177 caliber pellet will work in your .177 Gamo.
No it will jam in the barrel.
The 22 is slightly larger than the .177 cal. The 177 Pellet is almost the same diameter as the .175 BB.
Your best bet is to go on-line and do a search for "Airgun repair" and locate a shop near you for repairs.
Final Match Pistol Pellets by H&N
Any .177 cal pellet can be shot from you air rifle. However you will find that some brands are much better than others.
Somewhat. The .25 cal is just bigger and hits harder. Other than shape and air pressure behind the pellet, the size and weight of the pellet has a effect on the accuracy. OKAY look at it this way. (Pellets come in different weights.) a lightweight pellet will fly faster and farther than a heavy pellet. BUT it is more effected by crosswind than a heavy weight pellet. The Heavy pellet will fly straighter, slower and have more knockdown power than a lighter pellet. So a .25 cal pellet is heaver and slower than a .22 cal pellet but it has more impact and a little more accurate than a .22 cal pellet. PCP rifles come in different sizes. like .177, .20, .22, .25, 9mm, .357 cal 45 cal and 50 cal. Hitting a target with a .177 cal is like hitting it with a BB, but hit it with a 50 cal and you rip a huge hole in the target. The .177 will fly a lot farther but the 50 cal is a destroyer. I own both a .22 cal and a .25 cal Marauder. along with several others.
It all depends on the rifle, pellet and power source.
It was made between 1976 and 1983.