Not too sure about GMP. But when I joined I had to give confirmation that I had left the TA, I was then allowed to re-join after my probation. As I say, Not too sure about GMP but apparently only 0.5% of the workforce are allowed to be in the Volunteer reserves at any one time. With that all said Its now been 10 years and I applied to the job a couple of months ago and have been on a waiting list to get approval from work to rejoin. As and when they do it will be on a 5 year agreement when is reviewable.
As MSR said. You will massively need to reduce the TA commitment in the first two years. And knowing how the job works, you will need a good diary to keep shifts and 'weekends' on the straight and narrow.
My advice, check with GMP what their stand is in relation to joining whilst being in the TA. If they are up to strength on their allowance of officers in the TA/RN Aux/ RM Reserve etc etc, they may ask/advise you to leave and reapply as I had to.
Speaking as a serving copper, in a force next door to GMP and a serving TA soldier. My force is very supportive of TA, to the extent I get 15 days paid TA leave a year. BUT I understand Police Regs state you cannot be a serving TA member during your probation. ( although this may be my force's standing orders, I just remember reading it a while ago).
The other problem you have during probation period is the job tell you when you can have your leave, ie you will be so busy training going on courses etc etc you can't just apply for annual leave that suits you....good luck.