I can think of two options.

If you want to glass outside first... Use hot glue to hold the deck in place, don't overdo it as you are going to have to break it loose afterward. Do a test on some scrap to see how much hot glue you need. Glass the outside of the hull first and use pins at the sheer to hold it to the forms afterward. The join tape will seal them. Then glass the deck while the hot glue is still in place.

Or do what I do... Glass the inside first, especially of the deck. This leaves everything quite flexible so that the halves will join up fine. Then glass the outside.

Consider your workflow options before taking this route as it may limit you in some ways depending on what you are doing for hatches, deck fittings etc.

Which ever way you go, there is usually some small amount of a gap to pull into place. It is the nature of the beast. As long as it is small it will come out ok.

In the picture the hull is glassed both sides but the deck is only glassed inside at this point.

Et