Gill Sans actually isn’t free. However, some weights of it ship with OSX. Which isn’t free.

For the serif: Check out the FAQ Free. Charis SIL is somewhat similar to Scala Serif—if you don’t take account of much more than the angled serifs. Bitstream Vera comes in matching Sans, Serif & Mono.

Then of course, Scala is rather unique—and it starts from $40 …