In common with many chess variants, this idea has been 'invented' several times, going back to 1827. It has taken the idea of captured pieces then forming part of your reserve from Shogi (hence the 'gi'), so that the game never simplifies. This variant is one of the few which work as  'serious' game.

In considering your 'candidate moves' you have to pay attention to your opponent's reserve, or you can find your combination hitting a dead end when he drops a piece in the middle of your attack. Or suddenly finding yourself trying to deal with a pair of passed pawns on the 7th rank.

Gambits are almost always less useful in chessgi. The point of a gambit is too sacrifice a pawn for development, but in this variant you would be then be two pawns down for only the same compensation! Clearly a case of diminishing returns.

More complex than standard chess, you have to keep considering you can drop pieces to cause problems for your opponent (and where your opponent can drop pieces to cause problems for you!). Keeps you on your toes!