Thanks for all the suggestions.........
Anyone reading this who's in the same boat, I plumped for D' Addario Pro Steel medium gauge strings. Bright, but that works well with the woodyness of the Thunder Jet's natural tone.
When I recieved the Thunder Jet it had flats on it, which didn't work for me at all, the rounds have lifted the tone nicely.
Word to the wise, don't put short scale strings on a Gretsch Thunder Jet. It's a shortscale bass, but requires Medium scale strings because of the break angle over the bridge. The flats that were on it were shortscale, and the silks overlapped the nut, thus damping the tone somewhat.
Lovely bass, top notch, looks a cool as f***. I'm looking forward to gigging it.