There are multiple reports of Mailsync 134 errors on trying to connect Gmail. There’s quite an extensive history, so see the bug reports. (I won’t be migrating the comments.)
I am experiencing the same error but I might have an idea about the cause. I have two computers with the exact same setup. Both running Arch linux. My laptop had the same issue until I travelled to London. When I was there I changed the timezone on my computer and shortly after I was able to add my Google account. Could this problem have something to do with the country or the clock (oauth)? I am still unable to connect my google account on my stationery computer located in iceland (GMT0). I am a programmer myself and with help I could provide you with some debug information.
I am also having the same issue while connecting to GMail. In my current location, it is also not possible to access gmail without using a socks5 proxy or VPN. I am using KDE Neon 5.21 User Edition. I configured the proxy settings from the system. I have also tried through VPN but the issue remains the same. Please find a way so that we can access the gmail in mailspring. This is the only issue that I am having now. Other than that, I am enjoying using mailspring.