I think that in all relationships there is a moment when you realise that you are no longer in the honeymoon phase. The key here is, this has made you unhappy: it may be that, whilst she is "right on paper," there is not enough to maintain the relationship.
At the risk of repeating a cliché, the key here is communication. You cannot assume that this is one-sided, as it is entirely possibly that she is having the same feelings, and that a vicious cycle has started, whereby you both feel unenthused, and take less care of yourselves and each other as a result, leading you both to feel even less enthused, and so on. There are ways to communicate kindly but directly, and nobody, male or female can be expected to read minds. Just say that you are feeling as though things have fallen into a rut, then ask whether she feels the same way, and what you can both do to change that. Your OP reads a little as though you feel that the rut is down to her: you talk about her nagging, and her weight gain, without discussing any changes in your behaviour. I am not saying that it's all your fault, but you could ask yourself how you have contributed to the situation, and if you are treating her with the same care and respect that you used to. You have to be able to show yourself to your partner at your worst, but that should not be the only face that they get. This cuts both ways, and is not intended to suggest that you are solely responsible, just that you need to ask for her point of view and try to understand it.
Mind games such as buying clothes that will not fit will only make things worse. I know that the suggestion was offered in jest, but, whether or not she figures out your intentions (spoiler: she will), you are crushing her self-esteem, which never helped anyone or any relationship. Boost her ego without lying to her: if there is an activity she has stopped that she used to enjoy, help her get back into it. Encourage her to exercise for fun and fitness, but not because "she has put on weight" and you want her to be sexy. Whatever she does, it has to be for her as much as it is for you.
Your relationship is not doomed, but you always need to keep having the "state of the union" talk. Are you both happy? What could each of you do to be happier in yourselves and support each other being happy. At the end of the day, you need to avoid cruelty and Jedi mind tricks. It may not work out, but it might, and at the very least, you can leave each other better for having known each other.