No "obstacle" option in fluid type I'm trying to make gas flow through a tube.
I've watched many videos demonstrating similar procedure.
And they simply add a cube or any other object, go to the 'physics properties', click 'fluid',
then choose type as 'obstacle'

In their screens, the options are "Control, Particle, Outflow, Inflow, Obstacle, Fluid, Domain, None".

With my Blender 2.82, if I follow the same procedure, I see "None, Domain, Flow, Effector".
Where is it?
That would be the new updates in Blender 2.82 involving mantaflow etc.

I'd suggest that you go look for Mantaflow tutorials such as this one by CG Geek:

However, here's a quick summary of the four options that you have in 2.82:

None - Nothing.

Domain - The area that the fluid simulation stays within. None of the smoke/liquid will ever leave this area.

Flow - This is the actual smoke/liquid/fire emitter. You can change the type and additional settings within the settings of the Flow type.

Effector - These are your obstacles to the liquid simulation.