Yes, the GL.iNet firmware works just like the OpenWrt firmware as far as packages (
opkg) and upgrades (
sysupgrade) go, either over the command line or through LuCI, and they support a full set of package archives. The GL.iNet firmware supports "repeater" operation out of the box.
The OpenWrt builds, at this time, only support NOR flash. 16 MB of flash is generally sufficient for users that aren't installing huge packages (programming languages, fancy media servers, ...).
OpenWrt builds that support NAND flash for the AR300M and AR750S are hopefully not too far off, though I have to get the driver for some older-production units' Paragon-brand flash through upstream Linux. Once accepted, NAND-based builds should be available through OpenWrt downloads.
Until that time, local builds can be based of the branch
ath79-nand-2019-06-11 (or subsequent, later-dated revisions).