My team is
Bellsprout Lv. 20
Hoothoot Lv. 19
Quilava Lv. 21
Slowpoke Lv. 18
P.S. Miltank's Stomp is to strong
Geodude might be better than both Machop and Gastly. It has higher physical defense and resists all of Miltank's attacks, and it learns magnitude at level 16. Ground also ends up being a more useful type than fighting, and the fact that poison and ghost attacks are all physical means Gastly is very bad at using STAB attacks, so Geodude will be better than Machop and Gastly later in the game.
There are several ways you could go about this: