By default, the heading (title) always has a default margin, so you will see a space between the title (heading) and subtext (paragraph), like the screenshot.

To reduce this space, you can use below CSS code
#1. All text
Use this code to Custom CSS box
h1, h2, h3 {
margin-bottom: 0px;
}
h1+p, h2+p, h3+p {
margin-top: 0px;
}


#2. Specific Text
First, you need to find the ID of the Block. Use below tool.
In my example, we will have the ID:
- #block-yui_3_17_2_1_1722898813189_4140

Next, use this CSS code
#block-yui_3_17_2_1_1722898813189_4140 {
h1, h2, h3 {
margin-bottom: 0px;
}
h1+p, h2+p, h3+p {
margin-top: 0px;
}}
#3. Space on Mobile only
Use this CSS code
@media screen and (max-width:767px) {
h1, h2, h3 {
margin-bottom: 0px;
}
h1+p, h2+p, h3+p {
margin-top: 0px;
}}
#4. Space on Desktop Only
Use this CSS code
@media screen and (min-width:768px) {
h1, h2, h3 {
margin-bottom: 0px;
}
h1+p, h2+p, h3+p {
margin-top: 0px;
}}
#5. Space on One Page
First, you need to find the Page ID. Use tool in #2.
In my example, we will have ID:
- #collection-66aef2caff007776a825a948

Next, use this CSS code
#collection-66aef2caff007776a825a948 {
h1, h2, h3 {
margin-bottom: 0px;
}
h1+p, h2+p, h3+p {
margin-top: 0px;
}}