Its just a app. click on it to install it. When you run it, it will give you the option to make a backup of your sms and texts. I thought that was your main concern. This way you can copy all your sms, and then after backing up your contacts and such, you could do a factory reset to get your phone back to working order.
If I misunderstood what you were inquiring about, I'm sorry.
If you don't have the market on your phone, then the only way to install apps is to hook your phone to your computer and side load the app to you SD card. Then install the app from the SD.
I may not have the best understanding about how your phone works. When I said side loading, I just meant connecting your phone to your computer, dragging the app over to your SD card, and then installing the app from the SD card. On my phone, it's just that simple. Yours may take more work. I don't know if the G1 has a removable SD card, if so, you could try a card reader to do what I'm talking about.
If your just interested in getting the market on your phone, you might try a google search for the market.apk. Same installation like above.