I invested 800 gems for masteries for my Kael which was my first 60. It really helped me in maximizing my training for 12-3 or 12-6 and the Clan Bosses.
If you plan on playing the game long term, you want to maximize your Market slots and maximize the Mine.
You shouldn't use gems for expanding your Champion slots.
Don't buy ancient shards until there is an event which gives you bonus for them.