After testing this, the trick to throwing gas cans involves shoving the gas can as soon as you throw it:
- Step 1: Obtain a gas can.
- Step 2: Put your crosshairs at least 45° from the ground. I had better results the higher my crosshairs were angled.
- Step 3: Move forward, then throw it by pressing MOUSE1. Don't press E because you'll just drop it.
- Step 4: As soon as the gas can is released in front of you, shove it by pressing MOUSE2. The shove causes it to launch further.
The timing between MOUSE1 and MOUSE2 is about a tenth of a second. You'll know you did it right if you see the gas can more "flung" into the air (meaning it spins a bunch).
Jumping is optional, as it adds a little height to the throw, but doesn't contribute to distance nearly as much as shoving the can.
A note to those who want to practice this: don't do it on survival. The shove doesn't work on gas cans in this game mode, and after you shove it three times, it explodes into a flamey-gasoliney mess.
To put it in a simplified graph that also doubles as a masterpiece work of art:
This illustrates the importance of aiming upwards vs aiming towards the horizon.