All of the goodness will have gone out of your grow bag by now providing you have not had any problems with Any fungus or diseases that might have gone into the soil then just chuck it on to your main plot and mix in – If you have had problems then I would bin it

Recent research by Gardening Which? shows that compost can be used up to 3 times, it should be fine for seed raising.

Biggest problems we have had when re-using Growbag compost is the poor quality of the material used and the number of tomato roots in it. By the time I had sifted out the roots and the huge lumps of compost, there was not that much left to use.

I repeat - garden soil is ground up rock. I recycle all my compost by mixing it with my Gerbil pet's chewed up cardboard, some Blood Fish & Bone Fertiliser, and Horticultural Sand. The plants I have moved into it are flourishing, some take 3 months, some 6 months, but they all love it.
I would never throw old compost on the garden now.

I always re-use old compost as well, for containers, adding plant food granules when I plant up. I agree with Owdboggy though, that the compost used for tomatoes is always full of roots!