I am planning to change transmission fluid on my HHR. I got a new pan and and a new gasket that I am planning to install. I came across a video that shows application of Form-a-gasket sealant to the new gasket. Is this really a necessary step to take?

Here is the video:

 

No you don't need extra stuff on the pan seal. If anything your can reuse the original gasket/seal.
Why are you changing the pan itself ? Is it damaged ?

Picture of the gasket/seal below:

 

Simple answer - no.

 

X2 simple answer no, be careful and wipe the OEM gasket of any residual transmission fluid

 

Why?

 

It has always been a good practice to wipe off the residual fluids , don’t forget the gasket gets soaked when you drop the pan. Wiping it will reduce the mess while your putting the pan back into place.

 

Gasket and pan should be clean,dry and free of contaminants when reinstalling the pan. A wet pan or gasket (with trans fluid or non cured sealer) has the potential to squeeze out the gasket when tightening the bolts down.
This might not be as likely with the HHR trans and it's pan and gasket design, but the dry gasket procedure is one I've always used over the years on many types of repairs.
Back in the day, I dropped hundreds of pans on GM Turbo 350's and 400's. Plus many Fords. Having a clean,dry pan and gasket was drilled into my head by my Master Tech instructor.

 

The old pan is damaged. The gasket is leaking a little bit on one of the sides. Nothing serious, but I might as well change everything and not worrying about it for the next 100k miles.
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I assume there is no correct tightening sequence for transmission pan bolts. I decided to use alternating pattern that I got from another manufacturer.
Does 89 ft/lbs sound about right for these bolts? Couldn't find any any torque values on Tightening Torque Values for Common Maintenance Items page.
Drain plug seems to be 28 lb ft, but last time I torqued it to good-and-tight torque spec and everything seems to be okay.

 

NO ! 89 ft lbs on the pan bolts and you're going to destroy the pan or the gasket. That sounds like the torque value for the mounts.
I would think the pan bolts would be no more than 20 ft lbs. Maybe less.