I did a bit of research trying to find out who makes these air rifles. Here's what I found.
See the link below.
Also Webley also has a model called AGS, But I do not think they are related.
Sheridan air rifles who is owned by Crosman Sheridans are 5mm air rifles
Diana makes Original Air RIFLES.
The only information that I could find is that Kral AV Industrial makes the Demon air rifle along with several others. The company is in Turkey, and has made air rifles along with real rifles.See the link below.
China
Daisy makes several different model Air rifles each one has a different price. The Red Ryder usually sells for $39 here in California
Both companies make fine air rifles. Your question is too general. It's like asking which is better Honda or Toyota without comparing two specific models. Crosman is a larger company and makes air rifles in all categories from inexpensive to costly. Gamo is a smaller company and makes air rifles in the medium to costly range. So trying to say one is better than the other is not a fair comparison. They both make fine air rifles. Now if you had two specific models then you could make a comparison.
There called C02 air rifles or PCP air rifles
ags
Almost every country makes rifles
AGS Entertainment was created in 2006.
In runescape, you cant get ags. But you'll get Armadyl hilt drop and you will have to join it with godshards to make ags. Or buy it from grand exchange
AgS is not a metal. It is a compound called Silver sulphide