AVR debug probes and open-source GDB servers are poorly supported in my opinion. Microchip’s Atmel Studio is the only stable proprietary solution, see Custom AVR XMEGA board - #11 by maxgerhardt.
Is it a ATTiny x5? Maybe PR Enable self-programming for attinyx5 by orukusaki · Pull Request #513 · buserror/simavr · GitHub would help. If not, please open an issue at simavr to get that program improved.