Well, the solution (as armandounc so unhelpfullly put it) is to eat more. It takes some planning ahead, but it's really not hard to pack some meals and snacks to-go for your day. If you have a fridge at work, you can take leftovers from the previous night's dinner. Or if you have to take something non-perishable, make a pb&j sandwich! Grab some fruit---a banana or apple is easy. Granola bars, trail mix, crackers, or whatever you want to snack on.
I'm also trying to gain weight, so I drink the Carnation Instant Breakfast shakes (the ready-made bottles, not the powder) because they're easy to bring with me wherever I go! You can try Boost or Ensure---they're pretty much the same thing.
And make time for breakfast in the morning, even if it means getting up early or taking it in the car with you. It starts your day off well! :)