I'm glad you found your gerbil again - it is so scary when you lose one, isn't it? My last gerbil Pippy (RIP) escaped the day I got her...I had been used to Casper who was so tame that he didn't ever run away from me and so I got Pip out and it didn't occur to me that she didn't know to stay with me, so she was away before I knew what happened. Like you I really felt like an idiot. She was under the bed and begind bookcases and everything. I spent a good 2 hours trying to catch her (lucky I knew where she was and was living in a one-room flat at the time) before I realised that I would have to outsmart her rather than outrun her. I blocked all the exits to the bookcases except one, where I put her cage. Within a few minutes she was back home and safe. It took me a long time after that to regain her trust - not surprising after I spent so long chasing her around. Lots of sunflower seeds and peanuts eventually won her over!
I hope that the other gerbils soon accept Alero back into the tank. How long was she out? Good luck xx