This is based on opinion. Please don't list it on a work's trope example list.
Tear Jerker / Gaunt's Ghosts
Only In Death is a particularly grief-filled book. The time when they find Gaunt, still alive despite his sufferings, blinded, and desperate to know if anyone else had survived at the hands of the Blood Pact would be a tear-jerker, if it weren't competing with the point at which Hark discovers Soric as a sanctioned psyker, cries, and kills him. Doubly so in that Soric is generous and good-hearted to the end: in face of Hark's horror at what the Black Ships did, he tries to reassure him, though he had clearly suffered torments, and even before he asks for death, he assures Hark that he had been trying to help them, not hurt them; he tells Hark he has nothing to forgive him for and warns him to do it properly, so he won't be punished; and finally he ensures that Hark knows Gaunt is alive.
The death of Colonel Colm Corbec, treacherously stabbed in the back by Lijah Cuu.
Then, I did not find Corbec's death at the end of Sabbat Martyr to be half as tragic as Soric's fate there.
In one of his earliest appearances in Necropolis, Soric gets chastised by another officer for employing suicidal tactics against enemy tanks. His tear-jerking reply:
Even nameless mooks get quite a few tear-jerkers in the Gaunt's Ghosts series. In Sabbat Martyr, in particular, a platoon of about 20 Ghosts found themselves stranded outside of an Imperial fortress. Half a million enemy troops were heading for them. Grimly, the platoon turned around to fightthe overwhelming enemy force. The last words ever written about them were as follows:
Nineteenth [Platoon] lasted seventeen minutes from the time the gates closed. They accounted for one-hundred and eighty nine enemy casualties. No one witnessed their heroism.
In the Gaunts Ghosts novel 'Straight Silver', there are two true Tearjerker moments:
Once when Trooper Gol Koela, who had been brain-damaged in a previous battle seemingly gained fleeting recollection of his children, causing his loyal friend to gain hope that his memories are returning, only to reveal that Kolea was making a cold observation on the age of the P.D.F soldiers with him.
The second heart-wrenching moment was the death of Trooper Piet Gutes. He spends much of the novel reminiscing about his wife and children, killed back on their homeworld, and adopts a world-weary attitude for the majority of the novel. When he and a squad are defending an isolated, abandoned cottage in the forest, he rushes to defend a hole blown in the wall of the living room. As he runs out of ammunition, he sees a vision of his family, comforting him and assuring him that he doesn't have to fight any more. He settles-down into an armchair and is shot with his eyes closed and a smile on his face.
The entire sequence where Mkvenner and Kolea are healed by the Beati Sabbat in Sabbat Martyr, but particularly when Kolea looks to Sabbat, tears in his eyes, and pleads for her to "make [Ven] better". Kolea carried his bed-ridden comrade down Emperor knows how many flights of stairs because he had heard the waters cured all wounds, and he wasn't about to take "no" for an answer.
Immediately thereafter, when Kolea returns to the Ghosts' billet, their banter is at first Actually Pretty Funny ("I think she mostly healed Ven, I just got in the way.") with some barbs tossed around between friends (Varl: "Do you think she'd heal me?" "She only heals the deserving."), but it takes an abrupt turn for the serious when the terribly scarred Chiria asks if the Saint would heal her. All Kolea can think to say is "I'll ask."
Whenever Larkin sees visions of Bragg in Only in Death.
Merrt and Dorden's Last Stand in Salvation's Reach. They're both mortally wounded and decide to stay behind to buy their comrades time to retreat. It's particularly heart wrenching because they were two of the few remaining Tanith veterans.
Gaunt found Sar Af talking to Dorden. The old medicae looked especially fragile beside the vast Space Marine in his heavy boarding armour
'He is dying,' said Sar Af to Gaunt, as though this was news and come up in conversation.
'I know,' said Gaunt.
'But he is not afraid,' said Sar Af.
'I'm not,' said Dorden.
The White Scar nodded sagely.
He looked at Gaunt.
"And they shall know no fear," he said
This significance of this is hard to over-emphasise. Sar Af is quoting the Emperor, and in doing so directly comparing Dorden to the Astartes. There is no higher compliment.
The very concept of the Ghosts and their name. They originally got it because of their skill in woodcraft. Now they also have it because Tanith is gone, destroyed by a Chaos warfleet. Imagine, going off to fight a war knowing you have no home to go back to.
Vervunhive. Many of the Tanith members resented Gaunt for fleeing their homeworld when it came under attack on the day of their founding, even if the more reasonable among them acknowledged they couldn't hope to win and that Gaunt had fled so that they could make a difference later. And while Vervunhive had to be abandoned, the assault by Chaos was stopped, many of the people were safe, and the Ghosts were finally able to make their stand. And many of the survivors would become Ghosts themselves, forever joining the regiment that didn't abandon them in their hour of need.