The min requirement will include being employed by GB for at least 12 months. This scheme, I believe, is designed to give non flight deck employees a chance that does not come round very often. We have a lot of high quality people at the airline and this is a great opportunity for them, should they wish to apply.
Please also be aware that the selection will not be for the faint hearted as GB wants the very best candidates (as you would expect) irrespective of their present position in the company. Joining with a frozen ATPL and doing your 12 months' service will not guarantee you a position on the scheme.....indeed, you may well just waste 12 months of your life. A better idea would be to talk to personnel if you've already got a licence and they will tell you about the DEP situation.