Gen Urobuchi has stated that Black was a huge influence on Gaim - specifically, the duels between Black and Shadow Moon were what influenced the show's writing. Plus, Kouta is very similar to Kotaro, and Shadow Moon has a counterpart in this show as well (I cannot say who as that would spoil things).
Both shows have about 10 riders. These riders fight each other and also work together, depending on the situation.
Both shows have another dimension/world that is trying to merge with the world the riders live in.
The riders of Saber are swordsmen/knights. The rider suits of Gaim all have motifs of warriors, which include knights, samurai, and many others.
Both shows are a battle royal of good and evil Kamen Rider that are fighting to win a wish/fruit
Both shows have over 10 riders (if you include the ryuki movies).
Both shows have a dimension from where the power originates
The rider suits all have motifs of warriors, which include knights and many others.
If you like watching a young character slowly grows into the main villain like me, these 2 shows did it really well and similarly.
Both of these kids are loners overshadowed by the lead characters, and they never seem to achieve the same kind of popularity with people that the leads got.
Both also have mythical features (Heaven and Demon for Ashes of Love, Helheim Forest for Gaim) Both also have prophecy.
Both also come with many other grey characters.
The stories also progressed similarly. The kids all started out pretty innocent, which means both stories started out slow and comedic. When the stories hit a pretty similar turning point, you will know what you are really in for, and it will be serious from that point on. Several characters went through the point of no return. When characters reminisced, you will see that the stories have come really far.
Both pretty clearly have the split between the adults and the kids. The adults seemed all-powerful and kept the kids in check, but the kids are the ones fighting it out in the end. Both also have the fight between 2 brothers.
Romance is more prominent in Ashes of Love, but it also exists in Gaim, but this means many other prominent characters in Gaim are characters driven by several unique purposes, drowning out the romance. These characters have no equivalences in Ashes of Love and are interesting in their own rights.
Ashes of Love is much longer which allows more developments and side stories (sometimes recycled), but Gaim concisely achieved that main storyline. Ashes of Love is better than Gaim in terms of gender representation.
I know these 2 shows have different audiences, but seriously, they are so similar. If you finish one of them and can't move on, try the other one.
Both shows are a battle royal of toku heroes that are fighting to win a wish/power
The main characters are both naive and friendly
Suits are flash and colorful
Both feature a lot of rider on rider fighting and explore similar themes of growing up with many character having their ideals challenged
Dive is the series following Gaim so if you want to watch the Kamen Rider seasons in order then watch Drive after Gaim.
Gaim is the series after Wizard so if you want to watch the Kamen Rider season in order watch Gaim after Wizard.