You can use player-to-player (P2P) messages to send arbitrary data to other players. For example, you could do something like this:
gms_p2p_send(p2p_draw_card, other_player_id, "My Card", 4, 22)
Where p2p_draw_card is defined as a constant with a value between 0-255, other_player_id is the ID of the other player, and "My Card", 4 and 22 are the values that you want to send.
Note that while you could technically directly write the number instead of adding a constant (for example, p2p_draw_card), it's better to use a constant so you can assign a logical name to the number and use that instead.