Is XTS always better than GCM? XTS is suitable for disk encryption while GCM doesn't.
Both require the same number of block encryption calls.
Both are parallel in encryption and decryption.
XTS is actually even faster in openssl implementation (openssl speed -evp aes-256-xts/gcm), not sure why.
Why even bother to use GCM instead of XTS?
You're trying to compare authenticated encryption with disk encryption. GCM is used to solve 2 security problems at once: encryption and authentication. GCM is designed such that only someone knowing the key can modify ciphertext without raising alarms. This requires additional data to use as an authentication tag.
XTS, on the other hand, only partially hold this property. The benefit of XTS, however, is you don't need to store additional data for an authentication tag. XTS also has (some) resistance to tampering, since any change made to the ciphertext yields random and meaningless plaintext. However, this is not considered sufficient for authentication, since an attacker can still make changes without the changes being noticed. This is undesirable in any case where an attacker may intercept encrypted communication (ex. network protocols).
For full-disk encryption (FDE) authenticated encryption becomes impractical, since you would need to store a lot of extra data to verify integrity (with a mode like GCM). Unless you have so much disk space you don't mind losing half of it, this isn't a reasonable option. You could also store a separate hash (HMAC) but this still requires reading the entire contents of the disk to verify (which takes a long time), and will likely cause the hashes to mismatch simply due to normal wear and tear on the drive... Effectively creating a game of Russian Roulette; except instead of getting shot you lose data.
As such, unauthenticated encryption becomes the only rational way to encrypt an entire drive. Since an attacker cannot control the result of changes in XTS it's better than modes like CBC (where an attacker can rewrite the drive to whatever they want simply by being decently clever) or CTR (where cleverness isn't even a prerequisite to tamper with data), and XTS isn't vulnerable to statistical analysis like ECB. Therefore it's a not-ridiculous trade-off for cases where authentication can't be reasonably performed.