Hi everybody,
I recently completed the implementation of a GDB server for debugWIRE MCUs (e.g., classic ATTinys and small ATmegas) including the ATmega328P. This is now available as part of the MiniCore and MicroCore packages. If you want to debug a classic ATtiny, try my fork of ATTinyCore. Simply add the following additional board URLs in the Preference dialog:
https://felias-fogg.github.io/ATTinyCore/package_drazzy.com_ATTinyCore_index.json
https://mcudude.github.io/MicroCore/package_MCUdude_MicroCore_index.json
https://mcudude.github.io/MiniCore/package_MCUdude_MiniCore_index.json
As a hardware debugger, you can use all of Microchip's EDBG-based debuggers (JTAGICE3 (rev 3 firmware), Power Debugger, Atmel-ICE, PICkit4, SNAP, and mEDBG). In particular, SNAP is very attractive because Microchip has lowered the price to US-$ 15 recently. If you do not want to buy a debugger, you can also use the dw-link firmware and turn an Arduino UNO R3 into a hardware debugger.
How to connect your debugger to a target board is sketched in the README of dw-gdbserver.
Happy debugging!