Disclaimer: I am opening a huge can of worms saying so but it needs to be said!
Summoner situated in normal exp parties is best off being a main or back up healer, even after level 70, and yes even after level 75.
Until you reach level 42 where Regen and Cure III finally become available to you, you will have a real rough ride ahead of you.
Summoner is a real fun job, but be prepared, it is one of the hardest jobs in this game to level.
Now on Topic:
Yes, invest in that MP gear, especially on lower levels you will need it as you blow through it fast.
And as Galka you don't have much of that to spare, but you have the HP and natural VIT and DEF to safely equip Astral rings without risking being one shot by mobs in the case you happen to grab hate from excessive curing.
But next to MP gear like on any mage job, it is more important to factor in the speed in which you get it back.
HMP is one of the most important stats regardless of how much or little MP you have.
Having 300 MP in the dunes won't do squat when it's gone.
Until level 50 don't invest in summoning magic skill gear, there is only one option at level 35, the summoning earring which gives +3 skill but is in my opinion absolutely not worth the price asked for it and +3 skill alone won't do anything.
Even if you keep your skill capped.
At level 50 do invest in Austere/Penance gear!
(Although Penance Robe might be a bit far off, that thing is extremely rare, thus extremely pricy!)
Even if you don't have capped skill it will certainly help you with your Elemental Siphon MP return.