Sorry for late answer.
Even you had 20k diamonds from gift, do not waste it.
Believe me, you need alot diamonds for buying stamina, chance to challenge story stage and farming material for upgrade your Gundam skill.
My advice : if you want to gacha, choose Regular Gacha. Because every 10x gacha, you can got 10 points for exchange it to UR Gundam at Exchange Shop and your points wouldn't expired.
This is my progress ( playing around 20 days ). And i still had 5k diamonds left.
Oh, i almost forgot. Save alot resources for Unit, Character and Support.