So this happened last night in my game and I just wanted to made sure I got all the rulings right

Player A is a Tyranid player (swarmlord) with the initiative token

Player B is a Chaos player (Kugath)

Player A commits to the final planet with their swarmlord warlord, their synapse unit the gravid tervigon (spawns a termagant as a reaction after it commits) and a single Termagant Horde (Reaction: After a combat round at this planet begins, put 1 Termagant token into play at this planet.)

Player B also commits their Kugath warlord and a nurgling with 1 damage token already on it to the final planet

The only unit already on the final planet is Player Bs Gleeful plague beast

Forced reactions go before reactions, so before the reactions can trigger for the synapse unit, the nurgling deals 1 damage to units that moved to the last planet (both to player A and B). However since player A has initiative they can control how the damage is assigned since it is assigned simultaneously. They assign the 1 damage to the Chaos player units first (Kugath) before assigning it to the Nurgling that moved with Kugath which kills the Nurgling and stops the reaction which saves Player A from the 1 damage to his units that moved to the last planet.

After that the optional reactions go off and the Synapse unit spawns a termagant at the planet and the spawnlord can't spawn any termagants since there is no adjacent planets.

Then after the command struggle, combat starts which sets off the forced reaction of the Gleeful plague beast and the Termagant Horde. Since forced reactions go first the gleeful plague beast does 1 damage to everyone and player A assigns the order in which that 1 damage happens this results in the termagant token spawned by the synapse unit dying

After that the optional reaction of the Termagant Horde goes off and a termagant spawns.

After all the reactions have played out an action window is opened for the players.

Did that play out right according to the rules?

PS- a Adamant Hive Guard can't stop Kugath from moving damage to a unit, so far nothing in the game can stop that damage from being dealt including indomitable and iron halo right?

The timing is almost perfect: the Termagant Horde triggers at the start of a combat round, while the Gleeful Plaguebeast triggers at the start of the whole combat phase (so there is an action window in-between). The timing is:

3.1 Combat phase begins -> Gleeful Plaguebeast's forced reaction triggers

Action window

3.2 Battle initiates

3.2.1 Determine initiative

3.2.2 Combat round begins (with ranged skirmish, since it's the first of the battle) -> Termagant Horde's reaction can be triggered

You are entirely correct about moving damage.

Moved damage bypasses all damage prevention and reassign opportunities, and is placed directly upon the card to which it has been moved.

Then after the command struggle, combat starts which sets off the forced reaction of the Gleeful plague beast and the Termagant Horde. Since forced reactions go first the gleeful plague beast does 1 damage to everyone and player A assigns the order in which that 1 damage happens this results in the termagant token spawned by the synapse unit dying

It is also worth noting that this is incorrect. When Gleeful Plague Beast damages more than one unit, there is no order for the player with initiative to choose because it is a single effect, triggering once doing all the damage. (The phase starts once, so there is only one triggering condition, and only one Forced Reaction to initiate and resolve.) All of the units go through the "deal damage" process together, at the same time (like resolving AoE).

This is different from the multiple units moving to a planet with a Nurgling because there, it is a single effect, triggering multiple times . (X number of units move at the same time, so there are X number of different triggering conditions, and therefore X Forced Reactions to initiate and resolve - one for each triggering condition.) What the player with initiative is choosing there is the order in which the Nurgling's Forced Reaction triggers to all the simultaneous, but separate, triggering conditions.

Then after the command struggle, combat starts which sets off the forced reaction of the Gleeful plague beast and the Termagant Horde. Since forced reactions go first the gleeful plague beast does 1 damage to everyone and player A assigns the order in which that 1 damage happens this results in the termagant token spawned by the synapse unit dying

It is also worth noting that this is incorrect. When Gleeful Plague Beast damages more than one unit, there is no order for the player with initiative to choose because it is a single effect, triggering once doing all the damage. (The phase starts once, so there is only one triggering condition, and only one Forced Reaction to initiate and resolve.) All of the units go through the "deal damage" process together, at the same time (like resolving AoE).

This is different from the multiple units moving to a planet with a Nurgling because there, it is a single effect, triggering multiple times . (X number of units move at the same time, so there are X number of different triggering conditions, and therefore X Forced Reactions to initiate and resolve - one for each triggering condition.) What the player with initiative is choosing there is the order in which the Nurgling's Forced Reaction triggers to all the simultaneous, but separate, triggering conditions.

Looks like our Hive Tyrant was right in his video on this subject.