Yes, there are quite a few. You can start by looking on popular fanfiction websites like Fanfiction.net or Archive of Our Own. Just search for 'female Gajeel x Natsu' in their search bars and you'll find a variety of stories with different themes and plotlines.
Often, there's the theme of self - discovery. As they get involved with each other, they learn more about themselves. Also, the theme of friendship - turned - love is quite common. They start as comrades or rivals and then gradually fall in love. For instance, in a story, they might start by competing in magical battles and then realize there's more to their relationship. And adventure is also a theme, with them going on quests together while their relationship develops.
A common theme could be friendship. Natsu and Gajeel have a complex relationship in the original series, and in fanfictions with a female Gajeel, exploring how she fits into that friendship dynamic can be interesting.
There could be a plotline where they get into trouble in a magical realm. They have to rely on each other to find their way out. Along the way, they face various magical creatures and obstacles, which test their friendship and loyalty towards each other.
One great fanfiction could be 'Soul Bonds'. In this story, it beautifully weaves the relationships of Natsu x Lucy and Gajeel x Levy. It shows how Natsu and Lucy's bond grows stronger through various adventures, and at the same time, Gajeel and Levy's relationship develops in a really sweet way. There are some really emotional scenes that make you feel more connected to the characters. Another one is 'Hidden Feelings'. It focuses more on the internal struggles of the characters in these relationships. For example, Lucy might be hesitant about her feelings for Natsu at first, and Levy has her own insecurities when it comes to Gajeel. This makes the story more complex and interesting.
Finding good 'natsu and female gajeel fanfiction' can be a bit of a hunt. You could start by looking at the reviews of other Fairy Tail fanfictions. Sometimes, readers will mention other great works in their reviews, and there might be some that involve natsu and female gajeel. Another option is to explore the bookmarks of popular Fairy Tail fanfiction authors. They might have bookmarked some interesting stories with these two characters.
One interesting fanfiction could be where female Natsu and Gajeel team up for a special mission. In this story, their different personalities - Natsu's fiery enthusiasm and Gajeel's more brooding nature - create a great dynamic. They might start off clashing but as the story progresses, they learn to work together and respect each other's strengths.
One common theme is the growth of the relationships. In many fanfictions, Natsu and Lucy start as comrades in Fairy Tail and slowly develop romantic feelings. The same goes for Gajeel and Levy. They learn to understand each other better, overcome differences, and build a strong bond. Another theme is adventure. Since they are part of a magical guild, their relationships often develop during various quests and battles. For example, they might protect each other during a dangerous mission, which deepens their connection.
You can try websites like Fanfiction.net. It has a large collection of various fanfictions, and you might be able to find some 'natsu and gajeel lemon fanfiction' there.
One good 'natsu and gajeel lemon fanfiction' could be 'The Unlikely Bond'. It explores their relationship in a unique way, with great character development.