Your grandparents are showing you disrespect when they say you're being silly. Firmly, in a friendly tone of voice, ask them to stop saying that. Remind them that you're the parent and you're learning about foods and your baby. Not knowing or disagreeing is not being silly. I praise you for asking questions. If you think it'll help suggest to your grandparents that you want to know more about their beliefs surrounding food and that you'll consider what they suggest and that you'll decide whether or not to feed your baby what they've suggested. Tell them you'd appreciate them respecting your role as the baby's mother by feeding her only what you choose to feed her. That you'll respect their role as grandparents who've had more experience by considering their suggestions and information about them while making food decisions.
I agree with not feeding sweets. Once someone, whether a baby or an adult, develops a taste for sweets they crave sweets. Sweets are rarely nutritious and need to be saved for special occasions. Ice cream is somewhat nutritious and acceptable in small quantities for an older child. A baby in in a high speed developmental space and needs to have her calories be packed with nutrition. Ice cream is not packed with nutrition per calorie. It's too high in calories.
It's hard to understand the reasons behind food choices without some study in nutrition. For that reason, I suggest that referring to your doctor and nutritionists as the source of your choices can be very helpful. Read up on what foods are best for babies. You can google infant nutrition on the Internet and learn enough to guide you in your choices.
If your grandparents do not trust "experts" fall back on the statement, "I'm the mother and I get the final choice." Say it confidently and with the expectation that they will respect your choices. They may not. Don't fight with them. Just keep repeating the same words and over time they may get it. Or not. Unless your baby is taken care of by them every day you will still be her prime source of nutrition.
Be sure to praise them for being your baby's grandparents. I'm a grandparent and my daughter and I had some rough times because I felt that she was discounting my role as a grandparent. We eventually worked it out and here are some ideas that worked for us. She learned to say "thank you for the information. I'll think about what you said." She would frequently ask me questions to clarify my idea and thus show me that she was listening. When she decided she would often tell me why she'd made that decision. If I were giving her baby ice cream, she would tell me that many experts believe sweets are not good for a baby to have and that she'd decided to not give her baby any sweets until later. Please honor my plan."
Families often do not have boundaries which allow this sort of exchange. If your grandparents aren't able to accept that you are the one to decide what your baby eats, and if it's possible, I'd try to not have her visit at meal time. If you're there when they offer her ice cream accept a bowl for yourself. Perhaps allow her to lick your empty spoon occasionally if that will prevent a face off with the grandparents.
If they're your primary caretakers while you're working or frequently gone, then you may have to step back if they won't accept your boundaries. Then you'll have to decide how important feeding your baby your way is important.
Can you compromise and allow them to feed her table food but not sweets and junk food? If they smash the food up, she'll be able to gum in with saliva and then swallow it with no difficulty. It's unlikely that she'll choke but if she does, she will still be alright. If she gags often then I suggest that her swallowing reflex is not mature enough to handle solid food. Your grandparents should understand that.
Above all, you are not silly! Expect your grandparents and everyone else to treat you will respect. Stand firm when you tell them that you expect respect and silly is a disrespectful word. You are willing to discuss food with them but only as long as everyone respects everyone else.
Most of my generation and all of earlier generations only ate table food. Baby food was either not available or too expensive. Keep in mind that a baby's main source of nutrition up until they they're a year is breast milk or formula. Eating solids, even when pureed, is just practice for eating more later.