Hi I have finished dawngard and do all crossbow missions but when u go to do
More missions it says ask gunmar when I do it doesn't come up anyone can help
Me please ??

You may be told to speak to Gunmar for a new assignment, but when speaking with him he will have no new assignment for you.

You might double check that you don't have a quest already for him. - However, this may be a result of having a quest active already.

Do u mean any quest active or a dawnguard quest active ??

un-check any skyrim quest you probably have active or side-quest related to main quest, and after see if you have any possible dawnguard quest active