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.
Any .177 cal pellet can be shot from you air rifle. However you will find that some brands are much better than others.
diameter
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.
It's not the speed of the pellet it's the size and the type of the pellet. Yes a .177 caliber pellet can kill a pig but it most likely would take several shots, depending on where you hit it. Also "Hunting or hollow point pellets" should be used. A head or heart shot is best. I suggest At least a 25 caliber pellet should be used. If you can afford a a good hunting pellet rifle then I suggest a .25 cal Marauder, a 357 Rogue or a .50 cal Sam Yang. But these are expensive and have devistating power because they at PCP class rifles. (PreCharged Pneumatic) There are a few .25 cal springers or Nitro gas like the "Benjamin trail NP XL" that are a better choice than a .177 cal pellet rifle. .177 usually are not recommended for hunting anything larger than a rodent.
The 22 is slightly larger than the .177 cal. The 177 Pellet is almost the same diameter as the .175 BB.
Cal .177
It has nothing to do with velocity. It has more to do with the size, weight and shape of the pellet. I recommend a .25 Cal or larger pellet. (Yes Pellet rifles come in different sizes) The smallest to the largest are .177, .20, .22, .25, 9mm, 357 cal, 45 Cal, and 50 Cal. I also recommend using hunting (hollowpoint) pellets. The heaver the better the knock-down power. (Pellets come in different weights.) Also check to see if your location allows hunting with a pellet gun.
Yes.
Final Match Pistol Pellets by H&N
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.