Both are excellent "real robot" mecha anime, and share a very intense and "realistic" feel towards the combat depicted. Both have a strong romance subplot, though 08th is minus the high school setting and related wackiness and also bit thinner on the humor.
08th is also a very good series to get into Gundam, it's set in the same universe as the classic Gundam series however it's more of a side story then anything else.
VOTOMS and Gundam are huge Sunrise franchises, so it is bound to be some similarities. However, I think VOTOMS is most similar to the 08th MS Team, since both anime share the love for gritty and realistic action, making them prime examples of real robot anime. Also, both of them have an 'impossible love' relationship and both are partially inspired by the Vietnam war.
Both series offer a futuristic but very real and convincing depiction of warfare, featuring utilitarian designs and grounded combat with an emphasis on tactics, a rare thing to see in the mecha genre even among real robot series.
...have stupendous animation, taking its time to show how the mobile suits work and how are destroyed. Also modernizing the designs of the original series.
...are short spin-offs of the one year war, in the universal century, depicting a minor front of the larger war, that is notable for the difference in scenario and equipment deployed.
...take a more realistic and gritty aproach to Mobile Suit combat, with emphasis in equipment, tactics and strategy.
...follow characters, and dynamics among them, in both sides of the conflict portraying in a relatable and human way.
...have romantic subplots, albeit managed in different ways.
...have people who are devastated by the consequences and destruction of war, and people who enjoy the fight and think only about winning.
...have nice people in Zeon.  read more
Both anime are Mecha and Space type anime where the people are trying to cope with surviving against a certain impending threat. The real similarity comes from certain characters getting isolated in space from their stations trying to survive until help arrives, from which they start developing feelings for one-another; this in turn leads to scenarios which start to affect the plot in major ways.
Both are military anime which focus on a group of commandos sent on a dangerous mission in a secluded area. Both have a semi-realistic depiction of war, with the main different being that Gundam has mecha elements, while Cat Shit One has anthropomorphic animals. Overall, both are action-packed and have a similar atmosphere.
Other than Gundam Wing and G Gundam which are popular, try out 08th MS Team and to an extent, 0083 Stardust Memory. 08th MS Team too like G Gundam is unique in terms of style and 08th MS Team is ground based whereas G was fighting. Drama, romance, interactions, tone, story twists, and action are both great. More than one Gundam is involved which is a squadron fighting to end the war if you like Gundams.
Another great Gundam show to add if you only like Wing. 08th MS Team too is a mecha show but on a different scale dealing with ground based war scale other than space. It isn't just about mechas but action pack hand in hand combat and weaponry with bombs, guns, and blades. Character interactions are great especially Shiro Amada and Aina Sahalin as both begins to fall in love and share the same sentiments as the war goes on even to the end. There too is drama which is more than Wing and a much more darker tone.
Both shows were directed and heavily influenced by the late Takeyuki Kanda, who worked on Dougram together with Ryosuke Takahashi. The 08th MS Team is well-known for it's realistic approach on war, the insight in the everyday life of the soldiers, the relationships between the characters . Dougram has it all and adds politics, drama as well as battle tactics. The animation is outdated, but Dougram manages to present 75 well-paced episodes, combined with a huge cast and a detailed story. While Gundam 0079 was the first anime to introduce the concept of real robots, Dougram was the first one to combine robots, war and politics into a great series. read more
Mecha, Romance and two strong protagonists, what's not to like? The signature Gundam series shares many themes with RahXephon but can fall a bit more on the "technical side" of robots rather than its romance. However, it's a great watch despite its age.
If you liked the realistic depictions of soldiers without superpowers and post apocalyptic scenario of Blue Gender, then Gundam: 08th MS Team is a must watch. An interesting conflict with many types of realistic weapons and mecha with a distinguishable romance story makes 08th MS Team a good watch.
Both OVA's are essentially side stories to the original tv series showing the frontline of the war. TheIr also both slightly more realistic than the TV series and focus on more grunt soldiers than powerfull Newtypes or Perfect Soldiers.