A gallon is a bit much but half of that would be normal...
Yeast multiplies as it makes beer so if your yeast was active it made more yeast.
If the yeast is good quality harvest it. Put it in a 2 liter pet bottle, leave in the fridge overnight, pour off the beer on top and bottle the sludge in a small bottle. It will last months and you can toss it in a brew instead of the dehydrated stuff.
For what is there now, depending on the type of fermenter, we use plastic down under and lose about 2 liters below the tap, most of it is yeast cake and rubbish anyhow.
The good news is that what is left is all beer, you will have a very small yeast cake left at bottling time so not much more is wasted.
There are varying opinions about racking, i don't do it as a rule unless i want to do it for a reason (adding fruit or dry hopping etc). It will not harm your beer to stay in the primary for 2-3 weeks or more, it will have a similar effect to racking in that in this time the yeast settles (finings will help) and the brew is clear.