Nice bike!
About the frame...
The GT2 frame comes in 2 flavors from gasbike.net
Note the tank is as big as the Skyhawk kits 2.5L (2/3 gallon) tank so plenty big.
The GT2-A for 2-strokes (it has the front mount) and the GT2A-S for 4-stroke engine kits, you can see the subtle difference in the lower front tube geometry to accept the 4-stroke mount plate.
Note the frame is not painted.
Sure you can rattle can spray paint it but if you are building with everything new to put on the frame consider your neighborhood auto body place to paint the frame and everything else you want.
I have a new customer and just ordered the 49cc 4-stroke HS 142F engine with 4G transfer case, a GTA2-S frame, an SBP 4-stroke shift kit, and this yellow rim/black spoke 3-speed wheel set.
http://cdn3.volusion.com/2k9xa.y2wqc/v/vspfiles/photos/X-wheel-shimano3set-7.jpg?1389096330
The auto painting place around the corner from me will do it for $100 or less, it varies by paint color with red being the most expensive but it's a multistage paint process with sealing for a finish like a car hood on every mounting part you want painted so keep that in mind if you go with a GTA frame as unlike a rattle can spray paint job, that finish will be still awesome years from now.
I am a big fan of powder coating, I did the computer service for a local powder coating outfit years ago and got to learn all about it so don't get wrong, the wheels were most likely powder coated and worth it, this is a 'shop small neighborhood business' thing for me.
Anyway I ramble, sorry for kinda sidetracking your topic and your beautiful build but this topic is 1/2 a year old, I just wanted to answer the questions about the frame you used and what it involves.