The KYB GR2 would be a little firmer than the Gabriel strut - just by going by other reviews floating around. As far as warranty concerns - that is cmpletely up to you. Personally, I would rather put on a better part and change it 60K-80K miles down the and pay to get new ones, rather than wait for a lesser part to die and bring it in for warranty exchange. Keep in mind, with poorly performing struts - sure you'll get a replacement strut for a the bad one - but what about the potential collateral damage to tires (uneven wear), other suspension and steering components (accelerated wear and tear), or even the potential for an accident from poor or unpredicatable performance. Not worth the LLT warrant, IMO.
Plus, if you plan a running a lower spring - want to match the damping of the strut with spring rates. KYB GR2 are considered a good OEM replacements, lttle firmer than the original KYBs on the Corolla. KYBs are also reputed to have better fitment on most cars - I can vouch for the KYBs, fitment was like OEM. I know some struts you have to bend a bracket or cut/shave a piece here or there to make it fit - not sure if that applies to Gabriel struts through. The concensus online is that Gabriel is a big name - well known, but unfortuantly, like other well know names - been riding their namesake, instead of putting out the quality products that initially got them there.