For those wondering about Mystery Gift spawn conditions, I found the following on one of those wiki sites...
Your highest level reached must be level 40 or greater (NOTE: after going infinite, I was receiving gifts as early as stage 8 so only applies to those who are just starting the game).
You have opened fewer than 3 Gifts in the last 30 minutes.
At least 1 minute has elapsed since the last Gift spawned.
If all three conditions are met, each monster or boss has a 1 in 30 chance to drop a Gift.
My math isn't the greatest but by my working, this could take 500hrs ++ of waiting every 30mins to go in & click'n'collect gifts, then wait around for the 30mins to refresh!!
edit: 100 gifts opened @ 27hrs gametime - could have been done faster but setting alarms to check for gifts gets tedious fast!