Raise the back of your tongue to touch the soft palate. Add vocal cord vibration, then release.

Americans pronounce gases as GA-suhz (/ˈɡæsəz/). Stress falls on the first syllable — keep everything else short and quick. You'll hear it in sentences like "The atmosphere is composed of nitrogen, oxygen, and other gases".
Record yourself saying "gases" and play it back. The mic stays on your device — nothing's uploaded.
2 syllables, 5 sounds. Tap a syllable to jump to its row, then explore each sound's mouth shape and how it's made.
Raise the back of your tongue to touch the soft palate. Add vocal cord vibration, then release.

Drop the jaw noticeably. Keep the body of the tongue low and forward, and don't let the back of the tongue raise toward the soft palate. Pull the lip corners back slightly, almost a starting smile.

The textbook way isn't wrong — it's just not how anyone actually says it.
Stress falls on the first syllable, not the others. Stretch GA — keep everything else short and quick.
Don't pronounce the first syllable too fully. The unstressed syllable reduces to a schwa — the lazy "uh" sound — in casual speech.