Yes and no... Since nobody stopped by to say hello and answer your question, I decided to do it

Here's the thing, there may be nothing wrong with your rifle and you won't be able to shoot slugs out of it. That is simply because no two different guns, (gas-springers in your case), of exactly the same model, are in fact the same, even if they've come from the exact same assembly line, or otherwise belong in the same category of guns, of a specific brand or manufacturer. The reason for that hides in the bore dimensions, which are nothing alike from model to model, except on maybe guns that are of utmost, highest quality and in the highest price ranges - Gamos are no such guns. The milling or perhaps boreing machines that are used to make such guns, such as your Gamo, are not particulary precise and will therefore not be able to make barrels in the same dimensions, each and every time. So, with that having been said, your barrel might like slugs or it might hate them, however if you're going to try another gun of the same model, it's quite likely that it'll like slugs, or hate them for that matter, depending on how close the dimensions of the bore are going to be...
Get yourself some slugs and try them out, you've really got nothing to lose, except a few pennies

By the way, is your Swarm Mag in .22 or in .177 ? I'd like to make a suggestion on the weight of the slugs, that I'd recommend you to buy... Also depending on which you've got available in your state. But personally, I'd kind of go for lower weight H&Ns in 21gr-25gr range, or perhaps for some lead-free options...