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.
diameter
As long as the pellet is .177 Cal then any .177 caliber pellet will work in your .177 Gamo.
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.
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.
Cal .177
It usually says right on the barrel. Single shot / single load pistols are pellet pistols. Most BB guns have a reservoir for the BB's. There are a few that shoot either a BB or a pellet. You will have to look at the instructions or the display box to see what it says. Pellet pistols are more accurate than BB pistols. Most BB pistols will say .177 BB. Pellet pistols will say .177 Cal and not say BB.
Any .177 cal pellet can be shot from you air rifle. However you will find that some brands are much better than others.
It all depends on the rifle, pellet and power source.
The medalist is a .22 caliber pellet pistol. You need to purchase .22 cal pellets not .177 cal pellets. you can find them at most large sporting goods stores.
Any air gun or rifle that uses a .177 Caliber pellet can use the Gamo .177 Red Fire Pellet. No matter who makes the air gun. The Caliber .177 is universal to all .177 caliber pellet guns or rifles.