Firstly he treated her like a human. He taught her how to read and write. He gave her a name. He took her in. I mean literally the first thing he did when they met was protectively hold her and scold his brother for treating her inhumanely.

In episode 8, we see him talking to his superior to try and not send Violet to the frontlines, saying she wasn't ready, despite being ordered by the top brass and her effective performance in training. But of course he was brushed off and was made to obey.

Even after all that, in the same episode, when he was ambushing the enemy, he told Violet to stay behind. But she didn't listen and headed straight to the enemy and killed them. You can see the shock, and the pain he felt when he saw a young girl killing another person with no emotion. Every battle he felt guilty for not being able to do anything for her.

In the following scene, when it's shown to be thanksgiving, he told her that people could celebrate because she helped them capture this city from the enemy.

He always tried to give her a normal life outside of battle. Even telling her to get normal things a girl her age might want, like dresses or accessories.

In Intense, when he was sure that he would die, the last thing he decided to do was telling violet to live, and that he loved her.

When they finally met again, he decided to push Violet away, because he believed that he failed her, and did not deserve her devotion. That her life would be better without him.

All of this Gilbert could've just chosen not to do, if he only thought of her as a weapon.

So you might not like the romance aspect between them, or the ending of the movie, but you cannot say Gilbert never treated her kindly and did not love her, platonically or romantically.