stuck on grub rescue

Erase the whole HDD or SSD, repartition, and reload.

Partition corruptions arise far too often, and are far too easy to trigger.

This Mac is probably old enough that Internet Recovery isn’t always a good option, which can mean building a bootable installer on another Mac, and using that to wipe the storage on the problem Mac. This if Internet Recovery was corrupted by the repartitioning.

Use macOS Recovery on an Intel-based Mac - Apple Support (Option-Command-R for Internet Recovery)

If Internet Recovery does not launch, you will need to use a second Mac:

How to create a bootable installer for macOS - Apple Support

Or work with somebody else or with Apple Support; with somebody that can help re-load this for you.

When working with partitioning: have backups of all partitions. Those with data you care about. Have a bootable installer, have a second Mac bootable HDD or SSD, or a Mac with Internet Recovery in firmware, or the backups and preferably a combination of these recovery paths.

Agree with @ Mr. Hoffman and will have to reload the entire OS from scratch after reformatting the Entire Drive.

1 - Shutdown computer and disconnect all external drive Except the newly created Bootable Installer.

2- Restart and immediately hold the OPTION key until the Startup Manager appears and choose the USB Drive. 

3 - It will present options >> Disk Utilities >> View >> View ALL attached Drives. 

4 - Choose the Upper Most Drive ( not the volumes indented and list below ).

5 - The drive normally is called Apple Media or Apple SSD - that is the drive to Erase and format as HFS Journaled with the GUID Partition Map. 

6 - Once that is done >> backup out of Disk Utilities and choose install macOS. 

7 - Follow the prompts and it may automatically reboot several time. 

8 - Upon a final reboot - Setup Assist will present with the newer version of macOS.

So you have generated a bootable installer and selected bootable devices at startup and booted it (as listed in the createinstallmedia article) and still end up in GRUB? Impressive. You might have to use Target Disk Mode to get out of that.

Booting into Linux isn’t going to help get macOS installed. If Command-R (and probably best) Option-Command-R at power-up doesn’t get you into macOS Recovery, you’re going to have to rummage for help with somebody that can build you that bootable installer, or help with Target Disk Mode from another Mac.

Posting that same image a fifth time will no more move this forward than will the Linux partition tool shown.

Get some local help for the bootable installer or Target Disk Mode, or check with Apple Support and see if they can get you something bootable.

Been a while since I have used GParted. From the LiveCD, can you mark the hfs+ Recovery HD /dev/sda3 as boot (or /dev/sda2 (Ronnie's-imac) instead of /dev/sda1? Do you have a Time Machine backup of your macOS (ronnie's-imac) before you installed Linux?

All suggestions to follow is predicated on have a Bootable Installer of High Sierra.

What will happen, while in Linux GPart and removing all partitions & volumes, now a single Partition. Still in GPart and format the Upper Most Drive as Fat 32.

Back out of the Live Linux and attempt the boot to Bootable Installer of High Sierra.

What happens when you power off the iMac, and then attempt to boot into macOS Recovery by pressing ⌘+R immediately after the power button?