How can i center a view in a guideline?

You need to constraint both the top and the bottom of the View to the guideline, like so:

<android.support.constraint.Guideline
    android:id="@+id/guideline"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:orientation="horizontal"
    app:layout_constraintGuide_percent="0.3" />

<Button
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    app:layout_constraintTop_toTopOf="@id/guideline"
    app:layout_constraintBottom_toBottomOf="@id/guideline" />

Using fix width or height

Need to use same Guideline for both top and bottom constraint like:

app:layout_constraintTop_toTopOf="@id/guideline_30"
app:layout_constraintBottom_toBottomOf="@id/guideline_30"

xml:

<android.support.constraint.ConstraintLayout 
    xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    android:layout_width="match_parent"
    android:layout_height="match_parent">

    <ImageView
        android:id="@+id/imgLogo"
        android:layout_width="150dp"
        android:layout_height="150dp"
        app:layout_constraintTop_toTopOf="@id/guideline_30"
        app:layout_constraintBottom_toBottomOf="@id/guideline_30"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        android:src="@drawable/splash_logo" />

    <android.support.constraint.Guideline
        android:id="@+id/guideline_30"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:orientation="horizontal"
        app:layout_constraintGuide_percent="0.30" />

</android.support.constraint.ConstraintLayout>

output:

Using width and height based on the ratio

• As you are using both width and height based on ratio so that it's not centered.
• But to get your desired result you can also set Guideline = 0.15 (0.3 / 2) and modify constraintTop and constraintBotttom

xml:

<android.support.constraint.ConstraintLayout
    xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    android:layout_width="match_parent"
    android:layout_height="match_parent">

    <ImageView
    android:id="@+id/imgLogo"
    android:layout_width="0dp"
    android:layout_height="0dp"
    app:layout_constraintDimensionRatio="H,1:1"
    app:layout_constraintEnd_toStartOf="@+id/guidelineV_75"
    app:layout_constraintStart_toEndOf="@+id/guidelineV_25"
    app:layout_constraintTop_toTopOf="@+id/guideline_15"
    app:srcCompat="@drawable/ic_launcher_background" />

<android.support.constraint.Guideline
    android:id="@+id/guideline_15"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:orientation="horizontal"
    app:layout_constraintGuide_percent="0.15" />

<android.support.constraint.Guideline
    android:id="@+id/guidelineV_25"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:orientation="vertical"
    app:layout_constraintGuide_percent="0.25" />

<android.support.constraint.Guideline
    android:id="@+id/guidelineV_75"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:orientation="vertical"
    app:layout_constraintGuide_percent="0.75" />

</android.support.constraint.ConstraintLayout>

output:

In ConstraintLayout, for centering the View over Vertical axis of any other View Component (e.g. Guidelines, TextViews, ImageViews, layouts, etc.), you can simply follow these two steps, in XML:-

1) app:layout_constraintTop_toTopOf="@id/componentId"

2) app:layout_constraintBottom_toBottomOf="@id/componentId"

This will Equally distribute the component's View in the Vertical center.

BONUS:-

For centering the View in the Horizontal axis, use

1) app:layout_constraintLeft_toLeftOf="@id/componentId" or app:layout_constraintStart_toStartOf="@id/componentId"

2) app:layout_constraintRight_toRightOf="@id/componentId" or app:layout_constraintEnd_toEndeOf="@id/componentId"