There is no special case for being "caught behind" or "caught by the wicket keeper", so the rule is the same as at any other time: if the batsmen had crossed at the moment the catch was taken, then the former non-striker takes strike. If the batsmen had not crossed, then the new batsman takes strike.
Specifically, see Law 18.12(a):
When a batsman is dismissed Caught [...] the not out batsman shall return to the wicket he has left but only if the batsmen had not already crossed at the instant of the incident causing the dismissal.