How many gallons of beer is this batch? Was this extract, mini-mash, or all-grain? If you did a mash what temps did you hit? Did you add any non-fermentable lactose, starches, or other adjuncts? What was the OG, and what yeast did you pitch?
Also it sounds like you're trying to make it conform to a style guideline, if so what style would that be?
Short of adding lactose or blending with another beer I don't really think that you'll be able to increase gravity, but answer some questions and maybe someone can give you an idea how to proceed.
1.025 would normally be a really high target FG for a beginner beer. Are you brewing a barley wine or something similar?
Wait - you hit your target gravity and then added a gallon of water?
Something is weird here...
Sounds like OP hit the target gravity for the partial mash then topped off with both water and LME.
Not sure where you're getting the 1.025 number from, neither your OG or your FG should be that, but as mentioned sounds like you are totally fine.