I've just spoken to the pensions people and this, essentially is what they said:
"If you're on AFPS 75 and take up an FTRS post within 30 days of leaving regular service, then you will have to pay back your gratuity as a gap of less than 30 days is seen as continuous service. You will of course still get your gratuity when you finally retire from FTRS, so you don't lose it or have it taxed. Any period in excess of 30 days is seen as a break in service and therefore does not affect your gratuity."
If I'm honest, I don't have much faith in the advice being offered. When I initially asked the question, the guy I spoke to - after going away to make enquiries - told me I would lose my gratuity, full stop. When I told him that was clearly not true, he went away and eventually came back with the above advice. Somehow, i'm still not sure.
This will really have an impact on me as I would like my gratuity now, don't really want to have a 30 day break of service and would like to start my new job as soon as possible.