Update: During Season of the Lost, the Grandmaster Nightfall difficulty will be available until February 22 when the season ends.
During a season of Destiny 2, there will be 6 total Grandmaster Strikes available. Normally these strikes will cycle each week; but towards the end of the season all 6 will be available giving you the option of which strike you wish to attempt.
Before attempting a Grandmaster Nightfall it's important to note that you will be severely handicapped. Enemies are going to be 25 power levels above you meaning they will always hit harder. To add to that, you will need to be within 25 levels of the Grandmaster before even attempting it; this includes bonus power gained from the seasonal artifact. Bonus power is acquired through normal exp gains from anything in the game.
To demonstrate: The current pinnacle power (as of season 15) is 1330. Grandmasters are 1370, meaning you have to be at 1345 to even attempt a GM. You do not have to have be at the power level cap to attempt a GM you just need to be at 1345. That being said, if you are over 1345 you will still be treated as if you were 1345.
Grandmasters, in addition to Legend & Master difficulty, do not have matchmakinng enabled so you will be required to find a fireteam of three. Unlike the other Nightfall difficulties, Grandmaster gives your fireteam a pool of lives that is shared. If your fireteam loses all lives or all three of you are dead then you will be sent back to orbit, requiring you to start the nightfall from the beginning. However, your team can earn additional lives by killing what is called a Champion.
Champions are present in the Nightfall Ordeal difficulties of Hero all the way to Grandmaster. They are a type of enemy that requires a specific mod to combat them.
The mods are as followed
Barrier: A mod that pierces barrier of a champion which will stun it. After receiving a certain amount of damage, Barrier Champions will erect a barrier that will make them immune to damage unless you use a barrier mod to destroy their barrier.
Stagger: This mod will produce a stagger-like mechanic after you ADS for a few seconds. This is used on Unstoppable Champions that have an aggressive behavior when compared to other enemies. Once staggered, the champion will be unable to move for a few seconds and will more vulnerable to damage.
Disruption: This mod is used on champions that have an erratic behavior known as Overload Champions. These champions are similar in behavior to the Unstoppable in that they will be very aggressive. Some will move faster while others will attack faster. These are by far the more aggravating of the champion types.
One thing to keep in mind is that the mods needed to combat champions can be acquired from the seasonal artifact. In addition, there are some exotics that will have an anti-champion mod intrinsically.
One final tip: if you ever decide to do The Glassway strike - don't. You'll thank me.