Comcast XFINITY → [Install] xfinity extreme 105mbps

why does comcast have to send a tech to install xtreme 105mbps internet?

They didn't on mine.
I set and watched my 6183 re-boot then was getting 124/24 when it came back up.

If you currently have service, then they don't "have" to, it's more of a "because we can" fee to try and prevent people fro signing up for the service. As long as your signals are strong and you don't have any noise on your line, then you don't need them to come out. I told the rep when I called to upgrade mine there was no point in sending anyone out. I have the equipment, it's hooked up and waiting to be activated. After going around a few times with them they finally just did it. No tech has ever come out since, never was needed.

ok so i upgraded from 50 to 105mbps. im using the techcolor tc8305 router. do i need a gigabit router?

The TC8305C has gigabit Ethernet.

Only if you want to use a router and use the TC8305 in bridge mode, then yes you will absolutely need a gigabit router.