Can you solemn a Tytannial summoned by a Gigaplant or is it the resolution of a card effect and is too late?

No, you can't Solemn that Tytannial in that situation. You can't use Solemn Judgment to negate a Special Summon that's made through a monster's activated effect. To clarify courtesy of Netrep:

"Solemn Judgment" cannot negate a Special Summon if it results from the activated effect of a monster card, like "Mystic Tomato", "Light and Darkness Dragon", "Van'Dalgyon the Dark Dragon Lord", "Marauding Captain", "Lonefire Blossom", etc.

Cool. I made the right call then. It's appreciated!

Yeah, you'd need to negate Gigaplant's effect or its Normal (1st or 2nd in giving it its effect), Flip, or Special Summon.