It is such a worry if you find there's no water, but he didn't die
So he was getting fluid from somewhere (veg maybe) or it had only just run out.
As the others say, don't beat yourself up over having had a bad week. These things happen. How old is Gus? They often start to feel quite bony when they get older, however much they eat - they lose muscle tone.
One thing I do when they get older and seem a bit thin, is give high calorie extras. I did some research on it a few years back and some of the highest calorie foods are pecan nuts and hemp seeds. The pecan nuts in moderation of course. I used to give a shelled half of one once a week. But the hemp seeds I'd put out a pinch every day on a separate little dish - they just lick them up and seem to like them. They also contain all vitamins and nutrients needed so are a good "tonic".
Porridge, as mentioned also, is a nice extra treat - a bit on a teaspoon. But if you're struggling with depression than that can feel like an extra burden, thinking of making up porridge etc. And extra things to do. So perhaps getting some hemp seeds and pecan nuts would be easier. You just put them out when you put the food out daily.
I also think it's a good idea to have two water bottles in case one stops working properly. I always used to have two but now instead I just check and change the water daily. That can be a good habit - if you change the water every day, you don't forget - whereas if you do it every 2 or 3 days it's easy to lose track or forget to check!
I am so sorry you're having a hard time and hope you're getting some help with the depression. Everything can feel like a huge effort - but routine can help sometimes as well. So maybe set a routine of changing the water every day - just the one little task. And also putting the food out once a day at the same time. They tend not to eat it all but I find that they only take new food and even if there's plenty in their bowl they leave the older stuff! So I top it up each day for two or three days, then throw away anything remaining and just put the usual amount out, top it up for two or three days again etc.
Assume you give Gus a bit of fresh veg daily? They really love that and do get some liquid from that as well.
If you're struggling to manage though, maybe you need some help looking after Gus - is there anyone you can ask to help feed and do his water bottle for a while? So you can then enjoy doing things with him on a good day while knowing he is getting what he needs in between.