The first mention of "6 Infinity Gems" is Marvel Team-Up 55. I believe I posted the exact panel maybe 6 months ago in this thread.
I'm not sure where the first individual gem is mentioned but MP 1 is probably as good a guess as any.
First Soul/Infinity Gem: Marvel Premiere 1
First Mention of Multiple Infinity/Soul Gems: Random Issue of Captain Marvel
Here's what Wikipedia states are the 1st appearances/mentions of the gems/stones. I know Wikipedia can be edited, but more than likely whoever posted this did their homework. I found this interesting:
But Marvel Wikia states SS44 as first appearance:
http://marvel.wikia.com/Infinity_Gauntlet_%28Item%29
I don't have Thanos Quest 2 to verify but I do have SS44 and it's there.
Are the gems in the gauntlet in SS44? That seems to be what the Thanos #2 reference is about.
Oh sorry, I misread it.
I thought it was about the Infinity Gauntlet which 1st app in SS44 rather than just the gems themselves.
Carry on.
But yes, SS44 also refers to the gems on pg 6 - but it's not their 1st appearance though.
Notice their color.
Here's the panel from Captain Marvel #45 that mentions six Soul Gems.
Helluva catch on the CM 45! I just read through Power of Warlock #1 and although there is a reference to the jewel on Warlock's forehead it's not referred to as the "soul gem". I've got a couple of nice copies so I was hoping for fortune and glory, but alas, I cannot attest that this is the case. It wasn't even a good read