It's not essential per se that there is a constant GPS fix if the station isn't being moved around. However, the GPS is necessary for setting the proper time and that is absolutely critical.
If this is a Raspberry Pi SG it likely has an attached clock with battery backup. So if the SG can acquire a GPS fix to set the clock (might have to take it outside), and the battery in the backup is still good, then you should be ok leaving it where it doesn't get a regular fix, at least for a little while. You might also find that it does manage to get fixes, but only every day or two instead of hourly.
It really isn't ideal to leave it somewhere where it can't get reliable fixes regularly though. Do you have an external antenna for the GPS? If that isn't doing the trick, one with a longer cable may allow it to reach past the roof.