The good news is, yes, you can use other gateways!
A good way is to look at TTN Mapper (I like the heatmap view), then look for gateways in your neighbourhood. You can click on them and get and idea if they are any good, I like to use the “alpha shape” feature to get an idea of their coverage.
If you find a gateway that looks usable to you, you can try to find more information about it, e.g. by it’s EUI. I found this post very instructive: How to contact a gateway owner?
Once you figure out who owns the gateway, you can contact them through their user page, e.g.
https://www.thethingsnetwork.org/u/bertrik
(use the “Contact” button)
You can ask them to make you a “collaborator” for example, so you can view traffic coming in through that particular gateway.
Also try to find out if there’s a local TTN community in your area, through https://www.thethingsnetwork.org/community