Is GarageBand free for my device?

Earlier devices you purchased will not have any effect on the free apps that come for free with a new iPhone, see this Support document:

GetApple apps for free on your iPhone, iPad, and iPod touch - Apple Support

https://support.apple.com/en-us/HT202794

GarageBand should be free on your iPhone, but not on the iPad, there is a footnote:

*To download GarageBand for free, you need a device that was activated or purchased on or after September 1, 2014.

If you look at the regular App Store page it will show a price, Have you looked for "Apps made by Apple" or at your Purchase history, as described in the Support article I linked to?

GetApple apps for free on your iPhone, iPad, and iPod touch - Apple Support

https://support.apple.com/en-us/HT202794

Are you using the same AppleID as you used when you first set up your iPad? Have you changed your AppleID to a different country? All this could be causing the freeGarageBand version not to be showing in your Purchases history.