For GDX on 6m at this time of year I think more than 1W ERP is needed. The cable loss to my V2000 vertical is at least 1dB as I use a run of RG58 coax. I was quite surprised just how few UK stations were active on the 6m band. Back in the summer months when Es was plentiful I used to copy and be copied by, GDX stations by tropo or plane reflection out to around 200km most days. It seems outside the Es season the band gets largely vacated at this time of year, on WSPR at least. The couple of active UK stations were not copied here and did not copy me. As Doppler from moving planes is a big problem on VHF WSPR, it can take all day to get a favourable reflection that actualy decodes.
So, I am giving up on 6m for now. Life is too short and I lack the patience. I could run 5W from my FT817 but don't want to risk blowing the PAs at full power for nearly 2 minutes.