What product marketing skill gaps does someone with a sales background have? And what skills would translate well?

Congratulations on considering a career in product marketing. I "carried a bag" (literally) and it was an invaluable experience.

The good news is that you have a ton of useful knowledge: customer needs, your customers' buying groups and personas, their buying process, and more. You probably have some true empathy for customer pains that your product can solve. Skills that you may need to build up:

- Writing. Lots of it.

- Critical thinking and analytical skills. Product marketers have to see beyond the deal and the customer to look at the bigger picture. 

-Cross-functional collaboration. Product marketing's success depends on many other teams doing stuff.

Skills you may need to develop:

- Understanding of marketing technology and measurement approaches. 

- Understanding market research and quantitative analysis