Only creatures you control when the first ability resolves will gain trample. Creatures that come under your control later in the turn won’t have trample. Lands you control that aren’t creatures also won’t gain trample, even if you use the second ability to turn them into creatures later in the turn.

Skarrg Guildmage’s second ability doesn’t affect the land’s name or any other types, subtypes, or supertypes (such as basic or legendary) the land may have. The land will also keep any abilities it had.