The scratching is her trying to dig and nest. Make sure the sawdust is nice and deep.
Could also be a sign tgat the tank is too small or is not stimulating enough. Tubes and tunnels and cocoons are fab for gerbils as they love to burrow. To also help with boredom, scatter her food around the tank and hide little treats for her to find. This will encourage her natural foragging (sp?!) Instinct. Gerbils dont do well on their own, even without the distress of losing a companion, so enriching her environment is important.
Another quick point, gerbils huddle and sleep together for warmth, so i would consider a heat mat or even just a hot water bottle under the tank.
The gnawing is because she needs something to chew on. Rodents teeth never stop growing so its important to give them things to chomp on to wear them down. Natural untreated wood, salt licks, and even the cardboard loo roll tubes!
I agree with floaty, our gerbil loves a toilet roll inner (as long as there isn't loads of glue on it). It's really cute to watch them going into a frenzy of gnawing. We have a lone gerbil, she gets a lot of attention from us, we'll mainly me, and is so cute.
My gerbils are pretty scatty and don't enjoy being sat on my hand or anything like that. They are just not used to that sort of attention. I have got them balls they go in regulary. They love a good run around. Good excersize for them and you don't have to worry about them getting under furniture or chewing on wires!
Our gerbil is also scatty and fast and won't just sit in our hands. We have a gerbil playpen for her to have a run around in, we can pick her up from it carefully. Our last lovely boy used to escape from the playpen, he would climb over the bars and have a great time running round the room, if he could have talked, he would have said "haha you can't catch me". We always did eventually. They are such cute little furries and very easy to spoil.
Cardboard boxes and tubes to keep them busy, esp now he is on his own. They are sociable little things so he might be freaking out at being on his own although pretty sure you couldn't now introduce another one.
My little lady gerbil is currently having a whale of a time with all the inside rolls from wrapping paper and an empty egg box!!