Is Spider Tanks free to play?

Spider Tanks is a free to play game. But if you want to earn while playing Spider Tanks, you must have NFT, or you just need to play as a pilot to earn.

Are there any pre-requisite to playing the game, such as metamask wallet, etc.

All you need to play Spider Tanks is a Gala Games account and wallet such as Metamask, Coinbase wallet, or OKX wallet to receive a token.

What advantage does pay-to-play or NFT purchasers have over free-to-play?

The advantage of NFT purchasers compared to free-to-play ones is that they can make money, in the form of SILK tokens if you win the game. Apart from that, the tanks you have will be stronger than the free-to-play ones because you can level up tank parts. Meanwhile, free-to-play players cannot make money, and must be a pilot (where you play using NFT tanks owned by other people, and share the profits) to get SILK tokens. 

Can free-to-play players be competitive or enjoy the game?

For free-to-play players, you can still enjoy the game, and stay competitive with other players by playing as a pilot.