Gaining

you can try any type of bread and butter, or sugar free chocolate

I'm in the same low-to-no sugar boat. Ideas I've had so far include chips, meat sticks, and peanuts. I guess going overboard on salt isn't great either, so I'll be watching this thread as well.

You should try almonds too, very fattening

Cheese is good, if it is not convenient to be cutting it up you could pre-slice some, or buy Laughing Cow or Babybel.

Peanut butter works (you just need a spoon and grab a big spoonful of it). I did just keep a jar in my desk drawer at one point.

Any kind of seed or nut, although some of those can be a bit messy.

Really dark chocolate is pretty low in sugar (not sugar free, but you are not getting much if you don't have too much chocolate), although some people don't like the bitterness of it.

If you have access to a fridge for snacks, high fat yoghurt isn't bad (plain, the flavored stuff is full of sugar). Or simply milk, the calories are not that high but it is very easy to consume calories (or if you have access to a private fridge, cream!)

Good luck!

Hemp hearts! 6 calories per gram and really good for you

All the different nuts and nut butters, though some of them can get pricey. But I like the variety.

You can make homemade nutella with whatever sugar substitute you're comfortable with--the recipes are all over the internet, and while not exactly the same thing, they're pretty good.

Some kinds of fruit are lower in sugar, like apples. Check a chart--they're a lovely alternative to a cracker as a carrier for cheese.

I do a lot of low sugar baking just by lowering the sugar content and then replacing half of it with a sugar substitute. Almost always works great.

There are some good sugar free chocolates around, although as someone else said, a lot of sugar free candies can give you real intestinal upset if you eat too many.

You can also make yourself shakes with cocoa powder,. hemp hearts or flax, some frozen fruit, and a sugar sub. Lots of recipes around.

Hope that helps!