How slow are we talking? All my GOG downloads through Heroic have always been as fast as my connection allows. It sounds like there may be another issue at play.

Is there a VPN involved?

I download my gog games through heroic (desktop mode) and have no problems with download speed. Typically then I add the game to steam through Heroic.

40gb game in 5hrs? Might want to check your your bases. Reboot/update deck, reboot wifi router/modem. Run speedtests, retry download.

It's fast for me through Heroic. Maybe it's another issue?

Been hoarding some GOG offline installers in my spare time.

GOG infrastructure is just not as robust, I think. I've had some luck downloading during non-peak hours (morning and afternoon weekdays), but it's still slow in comparison to Steam.

I've never used Heroic, so I can't say what to expect, but I've never had any issues downloading games via the GoG website.

Have you tried that? Maybe it's faster?

Though, honestly, I think i might just be some outage on GoGs side. If the downloads via the website are slow too, it might be worth talking to their support.

Hmmm could you download to desktop pc and set up a Syncthing share to your Steam Deck? Synthing will sync directly over a local network when possible so it shouldn't be too slow.