In theory, play "air soft games." But if the gun is .177 caliber (as many are) and are the "sniper" variety (velocity is too high to be sold to minors or used in games in UK, Canada and US), then you can put metal or plastic BB's into them and shoot varmints. Or shoot at watermelons or tin cans or stop signs.
Low velocity air guns sold in Canada are supposed to have an orange barrel. The one Bryer is holding (last year's Christmas present) in the one photo does not appear to have an orange barrel. But, if it was purchased by an adult with a PAL (or off ebay or whatever) and given as a gift, then I am guessing that was not exactly legal to do. I don't know Canada weapons laws except that in Canada and UK, orange muzzles are required for "air soft play." There are air soft tournaments through North America and in UK (and probably elsewhere), and people wear face and eye protection, and use plastic ammo.
But, people also go hunting with them, simply swapping out the plastic ammo for metal ammo of the same size. This one:
https://www.amazon.com/Gamo-Rifle-S...un-20&linkId=42ec27258f5058f9c26e739d030ea904
for example is way way above the muzzle velocity allowable for kids to play with in Canada or US or UK. But, you don't need a gun license to buy one in the US. You can buy it on Amazon (even in California).
People mostly use higher caliber air guns for hunting (you can check it out on youtube or just google it).
We've been given a few airsoft guns (all .177, which I didn't know until today and all "designed" to work with plastic pellets, but we have only metal ammo for them - the kid who gave us the guns used metal ammo). We use them for target practice. One of ours is pretty powerful and I would not want to be shot in the ear with it point blank, frankly.
Bryer was supposedly very specific about which gun he wanted. If it's one of the 1000-1200 fps "sniper" or "shot gun" style air guns, it could be used to harm something, perhaps even to kill (people hunt with them). These types of "airsoft" guns are above the legal limit in terms of muzzle power to be used in airsoft games (or to be sold to kids). Canada requires a PAL to purchase one (not sure at what fps that requirement kicks in). But they are available at swap meets throughout the American West, can be disassembled pretty easily, and packed in a pretty ordinary duffle bag.
Bryer and his air gun here:
https://browse.startpage.com/do/show_picture.pl?l=english&rais=1&oiu=https://images.dailykos.com/images/700484/large/BryerSchmegelsky-Gun.jpeg?1564158393&sp=ea6df27905c9da852c760d47b86d16eb&t=default
I don't think it was the murder weapon, but I am not sure that a gun was used as the murder weapon in the first place (they also had a knife).