When I try to boot the game, the laoding screen appears and then freezes after a couple second. Any suggestions to fix this?

Tried the waiting method: didn't work
Reinstalled it: didn't work
Rebooted computer: worked
Tried to play 5 hours later: didn't work

I am unsure what is wrong.

Drivers maybe? Some kind of malware? I bought the game last weekend, 9h and 29m played and had no problems. It loads fast, and plays smoothly.

I have the same problem on all of 10tons GOG releases (Tesla vs Lovecraft, Jydge and Neon Chrome).
The only work around i know is to launch the game from the installation folder as launching it via the GOG Galaxy Client seems to make the game freeze most of the time.

can you alt-tab to the game & have it work?

Not quite sure what you're asking here. You can alt tab out of the game and back in just fine when its working. If its frozen upon launch you have to kill it via task manager.

I should also note (at least with tesla vs lovecraft) that your achivements are still synced back to gog galaxy if it is running in the background. Your saved games may not sync until you try and launch the game with gog galaxy however.

Hope that helps.

Some games I have look frozen but if I alt-tab to something else & back they work fine. If I didn't alt-tab it would act frozen, sometimes even windows would tell me it's frozen, but alt-tabing worked.

EDIT: sometimes this also shows a windows dialog is stopping the game from loading. IE firewall, error message, etc.

Few questions:
1) Which operating system are you running?
2) How do you launch the game? Directly from the .exe/shortcut or from Galaxy?
3) If you launch the game directly from the .exe/shortcut, is Galaxy running?

As TheHappyFriar said, there might be a popup window or similar behind the game window waiting for your response and the game appears frozen. Please try alt+tabbing to see whether there is such a popup window.