@GabrielH if you are lucky you can get commisions (the random ones you get be playing stages) that have gems as reward, I doubt you will able to save enough to buy everything but at least you can expand your dock (which is the first thing you must do to get more ships), if anyone want to play as f2p the dock should.
but to anyone who want to play and its playing to buy gems, the first time you purchase gems you will recive twice the amount you purchased so it's a good idea to save and purchased the max amount first.
and also gameplay wise, if you buy gems your priority shoud be like this:
1) Dock (get it to 200 first there are around 170 ships in the EN server and 250 in JP and CH)
2) Rings (Married ships will get 10% stats boost and some ships have special wedding dress costumes)
3) Skins (these are only for cosmetics purpose, they don't give any advantage)
4) unlock rows for the dorm (not recommeded at all for f2p, highly optional), btw unlock rows for the dorm =/= expand dorm, to expand the dorm you need deco-coins.
outside of that anything else it's a waste of money. btw, one last advice:
SAVE YOUR CUBES, GOLD AND DECO-COINS FOR EVENTS some events have time limited ships
which are not added to the permanent poll (some are) and you will have to waste gold and cubes to build them (1500 gold and 2 Cubes per event build), just build 1 ship a day outside events and only spend gold to buy T4 boxes for equipment. and the deco-coins for limited time furniture for the dorm, try to keep at least 2000 deco coins.