Save the gift giving until near the end of your event. But allow enough time to read all the tributes out loud.
It's best to let the party warm up a little and let everyone get acquainted. Let them catch up with each other and get to where they are a little talked out. That's when they will most focus on the main event.
Its also good to build a little anticipation with the guests. Of course, this is a surprise for the guest of honor, so they don't even know its coming. But everyone else will know and will be anticipating it, looking forward to seeing what the reaction will be.
Also, after reading all the tributes, and possibly some intense emotions, people will be worn out and ready to leave soon after. So its good to leave the reading for last.
But make sure you'll have enough time for all the tributes to be read aloud. You won't want any guests who sent in a contribution to see other people leaving before theirs is read. They will feel disappointed.
And if people start leaving, there will be fewer people to interact with the guest of honor as they continue reading their tributes.
Also, if people start leaving you'll be tempted to stop the reading, which would be a shame.