Of these 4 Cantonese initials, /g- k-/ are pronounced like their Putonghua counterparts.
Note, however, the Cantonese /h-/ & Putonghua [h] are noticeably different. Attention must be paid in mastering this initial. There should be no problem for English speakers.
The Cantonese /ng-/ has no counterpart in Putonghua, making it hard to learn. Only practice can lead to mastery. The same can be said to English speakers.
1. The Cantonese /g-/ & Putonghua [g] are both voiceless (that is, different from the voiced (hard) ‘g’ of English).
2. The Cantonese /k-/ & Putonghua [k] are the same, which is more or less like the ‘k’ of English.
For English speakers, note that the difference between /g-/ and /k-/ is, again, one of aspiration. For example, the (hard) ‘c’ in ‘car’ is clearly aspirated; whereas the (hard) ‘c’ in ‘scar’ is not. Cantonese /g-/ is identical to the English unaspirated (hard) ‘c’.
3. The Cantonese /ng-/ is an onset consonant that is present in neither Putonghua nor English. It is, however, common in both languages as coda. The [ng] in Putonghua [kāng] & ‘ng’ in English ‘king’ are identical to Cantonese /ng-/.
4. While the Cantonese /h-/ & Putonghua [h] are different,，it is the same sound as the ‘h’ of English.
Since the Cantonese /h-/ & Putonghua [h] are noticeably different, beginners of Cantonese find it hard to master. For more, see ‘Mastery of Cantonese Articulation – Mastery of the /h-/ Initial’.
Since the Cantonese /ng-/ is not an initial in Putonghua (or an onset consonant in English), it is a characteristic of Cantonese. That said, Cantonese speakers of younger generations often pronounce lose the /ng-/ initial. Syllables that traditionally begin with /ng-/ are now begun with the vowel, making them fall under the null initial instead. For more, see ‘Language Change in Cantonese – /ng-/ Pronounced Like the Null Initial’.
As to aspiration, see ‘Cantonese Characteristics - Aspiration’.