So, as I've mentioned before, my internet comes from a 3G SIM card that I use in a USB modem. Each SIM costs $25 per month for 20gb (and $10 per gb after that).

When I download through the gog downloader or my browser's downloader (whether be FF or Chrome, I've tried this with both), it measures my data usage and subtracts it from my total data allowance as it should.

However, I've noticed that when I download using GG's thing, no data is subtracted from my total remaining data. The same happened when I downloaded Starcraft with the blizzard downloader. I've counted up al my GG downloads from this month, and that alone is 30gb, + my gog downloads this month which were about 15gb (i stopped to avoid going over the limit) - in total I estimate that I've used over 50gb this month, yet my 20gb SIM is still telling me I have over 2gb left.

I've asked some people I work with (both in the IT industry and in telecoms), and no one has been able to provide an explanation. The cards are prepaid, so there is no concern over them realizing their mistake and trying to charge me for it later, I checked their ToS and there are no circumstances in which I can end up owing them money using the SIM.

So, what I'd like to know is why this happens. The aim is that if I can work out why it doesn't count data usage from GG, I might be able to get it to stop counting data usage from gog as well, which for me would equate to virtually unlimited internet access.

Any thoughts or advice would be highly appreciated. Thanks in advance.

The GG downloader might be using an unusual port and/or protocol, although I have no good explanation as to why your sim card doesn't notice these transfers (it seems unnecessarily complicated and unsafe to program it to recognize only certain protocols/ports). You could try to install a really meticulous firewall, remove all permissions for the GG downloader, the GOG downloader, and the Blizzard downloader, and then start each of them, while noting down the actions that the firewall reports. Then look for things that GG and Blizzard have in common and that are different with GOG. This would be somewhat tedious work though, as these downloaders tend to do a lot (changing dozens of registry keys for instance).

Blizzard uses a special-build BitTorrent downloader. Some other services do the same thing, but I don't know specifically what GamersGate uses. GOG uses HTTP, even with their special downloader, so it gets metered properly.

Maybe your service isn't capable of detecting and metering BitTorrent traffic.