Does Underrealm Lich's replacement effect activate Gaea's Blessing's triggered effect?
Yes, Gaea's Blessing will trigger

701.15b Revealing a card doesn’t cause it to leave the zone it’s in.
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701.15d Some effects instruct a player to look at one or more cards. Looking at a card follows the same rules as revealing a card, except that the card is shown only to the specified player.

Revealed cards are considered to be in the zone they were revealed from.  Therefore, when Underrealm Lich's ability causes you to look at cards and then put them in the graveyard, they have moved from the library to the graveyard, triggering Gaea's Blessing.