Don’t draw attention to the fact she’s gaining weight. Make her feel comfortable and confident, compliment her appearance thought out the day without mentioning her belly. If you got the money, buy her clothes and jewelry she would like.
Like the others said make sure she’s stocked up with plenty of her favorite snacks to graze throughout the day. When you take her out on dates to restaurants she likes, order appetizers she likes even if she says she doesn’t want any (say they’re for you). Order takeout and surprise her with it. Every few weeks buy her a bunch of candies and deserts with some flowers.
A trick I’ve been employing lately is whenever my girlfriend texts me that she’s having a bad or stressful day at work, I send her $20 and tell her to buy herself something nice for lunch. This probably happens 1-2 times a week.
I think biggest thing is to just make snacking and overeating very easy and comfortable, and the weight should start to come on. Now when she does start to gain a lot of weight there will probably be a moment where she really notices it, most likely when she and passes into fat territory instead of just pudgy/chubby like she is now. If she responds negatively to it there’s not much you can do. That’s why boosting her confidence is so important so that she feels good about herself and when the fact she is now fat starts to hit she can just shrug it off.